Nautical Mile (NM) - Unit Information & Conversion

Quick Answer

1 nautical mile (NM) = 1,852 meters = 6,076.115 feet = 1.15078 statute miles

A nautical mile represents one minute of arc (1/60°) of latitude on Earth's surface, making it perfectly suited for navigation calculations using charts and coordinates.

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Quick Comparison Table

Length Unit	Equals 1 Nautical Mile
Meters	1,852 m
Kilometers	1.852 km
Feet	6,076.115 ft (approximately 6,076 ft)
Statute Miles	1.15078 mi (~1.15 mi)
Yards	2,025.372 yd
Latitude Minutes	1 minute of arc (1/60 of a degree)
Degrees of Latitude	1/60° (0.01667°)

Key Relationship:

• 60 nautical miles = 1 degree of latitude
• 1 knot = 1 nautical mile per hour

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Definition

A nautical mile (symbol: NM or nmi) is a unit of length specifically designed for marine and air navigation, officially defined as exactly 1,852 meters (approximately 6,076.115 feet or 1.15078 statute miles).

Why Is the Nautical Mile Special?

Unlike arbitrary land-based distance units (statute miles, kilometers), the nautical mile is geometrically derived from Earth's dimensions:

1 nautical mile = 1 minute of arc along any meridian (line of longitude)

This means:

• 60 nautical miles = 1 degree of latitude
• 1,800 nautical miles = 30 degrees of latitude
• 10,800 nautical miles = 180 degrees (equator to pole along a meridian)

Navigation Advantages

This geometric relationship provides critical benefits for navigation:

1. Direct Coordinate Conversion:

• If your ship is at 40°N latitude and sails due north to 41°N, you've traveled exactly 60 nautical miles
• No conversion factors needed—degrees and minutes directly translate to distance

2. Chart Plotting Simplicity:

• Nautical charts have latitude scales on the sides
• Measure distance by comparing to the chart's latitude scale at the same latitude
• One minute of latitude = one nautical mile (exact)

3. Celestial Navigation:

• When using sextants to measure star/sun angles, angular measurements directly convert to distance
• Essential for historical navigation before GPS

4. Universal Consistency:

• The nautical mile works identically at all latitudes (unlike longitude distances, which vary)
• International standard used by all maritime and aviation authorities

Nautical Mile vs. Statute Mile

Attribute	Nautical Mile	Statute Mile
Definition	1,852 meters (Earth-geometry based)	1,609.344 meters (historical land measurement)
Length in Feet	6,076.115 ft	5,280 ft
Basis	1 minute of latitude arc	Historical English mile (1,000 paces)
Primary Use	Maritime & aviation navigation	Land distances, road travel
Ratio	1 NM = 1.15078 statute miles	1 mi = 0.86898 nautical miles
Speed Unit	Knot (NM/hour)	Miles per hour (mph)
International Standard	Yes (since 1929)	No (U.S., U.K. primarily)

The Knot: Nautical Speed

A knot is one nautical mile per hour:

• 10 knots = 10 NM/hour = 18.52 km/h = 11.5 mph
• 30 knots = 30 NM/hour = 55.56 km/h = 34.5 mph

Why "knot"? The term comes from 17th-century ship speed measurement using a chip log—a wooden board tied to a rope with knots at regular intervals (typically every 47 feet 3 inches, or 14.4 meters). Sailors would throw the board overboard and count how many knots passed through their hands in a specific time (usually 28 seconds measured by sandglass). This gave an approximate speed in "knots."

Modern Usage: While chip logs are obsolete, "knot" remains the universal maritime and aviation speed unit. Ships' logs, flight plans, weather reports, and international regulations all use knots.

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History of the Nautical Mile

Ancient Navigation: The Seeds of Angular Distance (c. 300 BCE - 1500 CE)

Greek Geodesy (c. 240 BCE):

• Eratosthenes calculated Earth's circumference with remarkable accuracy (~250,000 stadia = ~39,375 km, only ~2% error from modern value 40,075 km)
• Established that Earth is spherical and could be measured in angular degrees
• Greek astronomers divided circles into 360 degrees, each degree into 60 minutes, each minute into 60 seconds

Ptolemy's Geography (c. 150 CE):

• Ptolemy created maps using latitude and longitude coordinates
• His calculations of Earth's circumference were less accurate than Eratosthenes' (underestimated by ~30%)
• This error influenced European explorers for over 1,000 years

Medieval Navigation (c. 1000-1500 CE):

• Vikings and Arab sailors navigated using dead reckoning (estimated speed × time) and celestial observations
• No standard distance unit tied to Earth's geometry yet
• Various regional distance measures: leagues, Roman miles, Arabic farsakh, etc.

The Age of Exploration: Linking Angles to Distance (1500-1800)

Navigational Revolution (16th Century):

• Development of portolan charts (Mediterranean sailing charts)
• Invention of cross-staff and backstaff for measuring celestial angles
• Navigators increasingly aware that angular measurements could determine position

The Sextant Era (1731):

• John Hadley (England) and Thomas Godfrey (America) independently invented the sextant
• Allowed precise measurement of angles between celestial objects and horizon (accuracy: ±0.1 minute of arc)
• Enabled celestial navigation: determining latitude by measuring sun's or Polaris's altitude
• Created practical need for distance unit corresponding to angular measurements

Emerging Nautical Mile Variants (1700s):

• British Admiralty Mile: 6,080 feet (based on British measurements of Earth)
• Various European Miles: Different countries defined nautical miles based on their estimates of Earth's circumference
• No international standard yet—created confusion in international navigation

The Problem of Longitude:

• While latitude could be determined astronomically, longitude required accurate timekeeping
• John Harrison's marine chronometer (1760s) solved this, enabling precise position fixing
• Further emphasized need for standardized navigation units

The 19th Century: Toward Standardization

National Definitions: By the mid-1800s, major maritime nations used different nautical miles:

• British Admiralty: 6,080 feet
• United States: 6,080.20 feet (slightly different Earth measurements)
• France: 1,852 meters (using metric system)
• Germany, Italy: Various slightly different values

Geodetic Improvements:

• Better measurements of Earth's shape revealed it's not a perfect sphere but an oblate spheroid (equatorial bulge)
• One minute of latitude varies from 1,842.9 meters at the equator to 1,861.7 meters at the poles
• Average: approximately 1,852 meters

International Trade and Navigation:

• Steamship era (mid-1800s) increased international maritime traffic
• Inconsistent nautical mile definitions caused practical problems:
  • Charts from different countries used different scales
  • Navigation calculations required conversion factors
  • International maritime law needed standard distances

International Standardization (1929)

The Monaco Conference (1929):

• The International Extraordinary Hydrographic Conference convened in Monaco
• Delegates from major maritime nations attended
• Goal: Establish universal standards for hydrographic charts and maritime navigation

The 1,852 Meter Standard: The conference adopted:

• 1 international nautical mile = 1,852 meters (exactly)
• This equaled approximately 6,076.115 feet
• Based on the average length of one minute of latitude over Earth's entire surface
• Compromise between various national definitions

Why 1,852 meters?

• Earth's mean circumference: ~40,007 km (at the poles and equator average)
• 40,007,000 meters ÷ 360 degrees ÷ 60 minutes = 1,852.0 meters/minute (approximately)
• Close to French definition (already 1,852 m), easing French adoption
• Reasonably close to British/U.S. definitions (minimizing disruption)

Rapid International Adoption:

• International Hydrographic Organization (IHO) promoted the standard
• International Civil Aviation Organization (ICAO) adopted it for aviation (founded 1944)
• By the 1950s-1960s, virtually all maritime and aviation authorities worldwide used 1,852 meters
• United States officially adopted it in 1954 (though U.S. Coast and Geodetic Survey used it earlier)
• United Kingdom adopted it in 1970, replacing the Admiralty mile

Modern Era (1950-Present)

Aviation Adoption:

• Civil aviation adopted nautical miles and knots as standard units
• Flight plans, air traffic control, pilot reports all use NM and knots
• Altitude measured in feet, but horizontal distances in nautical miles

GPS and Electronic Navigation:

• GPS coordinates use degrees, minutes, and seconds—directly compatible with nautical miles
• Modern electronic chart systems (ECDIS - Electronic Chart Display and Information System) use nautical miles
• Despite metrication in many countries, nautical mile remains universal for navigation

Why Not Kilometers?

• Some advocated replacing nautical miles with kilometers (metric system)
• Arguments against:
  1. Nautical mile's geometric relationship to latitude is uniquely valuable
  2. All existing charts, regulations, and equipment use nautical miles
  3. Aviation and maritime are inherently international—need consistent units
  4. Retraining entire global maritime and aviation workforce would be enormously expensive
• Result: Nautical mile remains entrenched, with no serious movement to replace it

Legal Status:

• Recognized by International System of Units (SI) as a "non-SI unit accepted for use with the SI"
• Defined in terms of SI base unit (meter): 1 NM = 1,852 m (exact)
• Official unit in international maritime law, aviation regulations, territorial waters definitions

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Real-World Examples of Nautical Miles in Action

1. Maritime Navigation and Shipping

Ocean Crossings:

• New York to London: Approximately 3,000 nautical miles
• At 20 knots (typical cargo ship speed): 3,000 NM ÷ 20 knots = 150 hours = 6.25 days
• Modern container ships (24-25 knots): ~5 days
• Historical sailing ships (5-8 knots): 15-25 days

Major Shipping Routes:

• Panama Canal: 40 nautical miles (Atlantic to Pacific)
• Suez Canal: 120 nautical miles (Mediterranean to Red Sea)
• Singapore to Shanghai: ~1,600 nautical miles
• Los Angeles to Tokyo: ~4,500 nautical miles

Ship Speed Classifications:

• Slow steaming (fuel efficiency): 15-18 knots
• Normal cruising: 20-24 knots (container ships)
• High-speed ferries: 35-50 knots
• Naval destroyers/frigates: 30+ knots
• Fastest ships (hydrofoils, hovercraft): 60-70 knots

2. Aviation Navigation and Flight Planning

Flight Distances (Great Circle Routes):

• New York (JFK) to Los Angeles (LAX): ~2,450 nautical miles
• London to Dubai: ~2,700 nautical miles
• Sydney to Singapore: ~3,900 nautical miles
• New York to Tokyo: ~6,700 nautical miles (polar route)

Aircraft Speeds:

• Small propeller aircraft: 100-150 knots
• Regional jets: 300-400 knots
• Commercial airliners (cruising): 450-500 knots (typically 0.78-0.85 Mach)
• Business jets: 450-550 knots
• Military jets: 600-1,200+ knots

Air Traffic Control:

• ATC clearances: "Cleared to 35,000 feet, maintain 280 knots until 10,000 feet"
• Flight plans specify distances in nautical miles, speeds in knots
• Separation standards: Aircraft must maintain minimum separation (e.g., 5 NM horizontal, 1,000 ft vertical)

3. Territorial Waters and Maritime Law

International Law of the Sea (UNCLOS - United Nations Convention on the Law of the Sea):

• Territorial Waters: 12 nautical miles from baseline (coast)

  • Nation has full sovereignty (like land territory)
  • Foreign ships have "innocent passage" rights

• Contiguous Zone: 24 nautical miles from baseline

  • Nation can enforce customs, immigration, sanitation, and fiscal laws

• Exclusive Economic Zone (EEZ): 200 nautical miles from baseline

  • Nation has exclusive rights to natural resources (fish, oil, minerals)
  • Other nations have freedom of navigation and overflight
  • Covers ~38% of world's oceans

• Continental Shelf: Up to 350 nautical miles (in some cases)

  • Nation has rights to seabed resources

Example: United States EEZ:

• U.S. EEZ extends 200 nautical miles from coastlines (including Alaska, Hawaii, territories)
• Total area: ~3.4 million square nautical miles
• Larger than the land area of the United States

4. Weather Forecasting and Meteorology

Wind Speed (Knots):

• Beaufort Scale (maritime wind scale):
  • Force 0: < 1 knot (calm)
  • Force 3: 7-10 knots (gentle breeze)
  • Force 6: 22-27 knots (strong breeze)
  • Force 8: 34-40 knots (gale)
  • Force 10: 48-55 knots (storm)
  • Force 12: 64+ knots (hurricane)

Hurricane/Typhoon Classification (Saffir-Simpson Scale):

• Category 1: 64-82 knots (74-95 mph)
• Category 2: 83-95 knots (96-110 mph)
• Category 3: 96-112 knots (111-129 mph)
• Category 4: 113-136 knots (130-156 mph)
• Category 5: 137+ knots (157+ mph)

Maritime Weather Reports:

• Marine forecasts report wind speeds in knots
• Visibility in nautical miles: "Visibility 5 nautical miles in rain"
• Wave height in feet or meters, but fetch (distance over which wind blows) in nautical miles

5. Naval Operations and Military Applications

Naval Combat and Strategy:

• Weapons ranges specified in nautical miles:

  • Harpoon anti-ship missile: ~70 nautical miles
  • Tomahawk cruise missile: ~900 nautical miles
  • Naval gun systems: 10-20 nautical miles

• Radar ranges: Measured in nautical miles

  • Surface search radar: 20-50 NM
  • Air search radar: 200+ NM for aircraft detection

Submarine Endurance:

• Nuclear submarines can travel 20+ knots submerged for months
• Distance: 20 knots × 24 hours × 30 days = 14,400 nautical miles/month (without surfacing)

Aircraft Carrier Operations:

• Carriers steam into wind to launch aircraft (need ~30 knots wind over deck)
• Carrier itself: ~30 knots maximum speed
• Launch/recovery operations: Position within ~50 NM of battle group

6. Search and Rescue Operations

Search Area Calculations: When an aircraft or ship goes missing, search areas are calculated in square nautical miles:

• Initial search radius: Based on last known position + estimated drift/movement
• Example: Aircraft disappears 200 NM from coast
  • Search radius: 50 NM around last position
  • Search area: π × 50² = ~7,850 square nautical miles

Drift Calculations:

• Ocean currents: 0.5-3 knots (depending on location)
• Wind drift: Objects drift ~3% of wind speed
• Example: 20-knot wind + 1-knot current = drift ~2 knots in specific direction
• After 24 hours: Object may have drifted 48 nautical miles from last position

SAR Response Times:

• Coast Guard helicopters: Range ~200 nautical miles, speed ~120 knots
• Response time to incident 100 NM away: ~50 minutes flight time

7. Recreational Boating and Sailing

Yacht Cruising:

• Daily sailing distance: 100-150 nautical miles (at 4-6 knots average)
• Atlantic crossing (Canary Islands to Caribbean): ~2,700 nautical miles
  • Average time: 15-25 days (depending on wind, boat speed)
  • Trade wind route: Favorable winds ~10-20 knots

Sailing Races:

• Transpac Race (Los Angeles to Honolulu): 2,225 nautical miles
  • Record: ~5 days (mono-hull), ~2 days (multi-hull)
• Vendée Globe (around-the-world solo): ~24,000 nautical miles
  • Record: 74 days (Philippe Jeantot, 2016-2017)

Navigation Equipment:

• GPS chartplotters display position in latitude/longitude and distances in nautical miles
• Paper charts: Distances measured using dividers against latitude scale (1 minute = 1 NM)
• Radar: Displays range rings in nautical miles (0.5, 1, 3, 6, 12, 24 NM settings)

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Common Uses of the Nautical Mile in Modern Contexts

1. Commercial Shipping and Maritime Trade

Virtually all ocean-going commerce uses nautical miles:

• Voyage Planning: Routes calculated in nautical miles, speeds in knots
• Fuel Consumption: Ships burn X tons of fuel per nautical mile at Y knots
• Charter Rates: Sometimes calculated per nautical mile traveled
• Port Distances: Official port-to-port distances published in nautical miles
• Shipping Schedules: Container ship services maintain schedules based on NM distances

Industry Standard: International Maritime Organization (IMO) regulations, SOLAS (Safety of Life at Sea) convention, and all maritime treaties use nautical miles.

2. Aviation and Air Traffic Management

Every aspect of aviation navigation uses nautical miles and knots:

• Flight Plans: Filed with distances in NM, estimated time en route
• Air Traffic Control: Controllers vector aircraft using headings and distances in NM
• Minimum Safe Altitudes: Calculated based on terrain within X nautical miles
• Separation Standards: Aircraft must be separated by minimum NM horizontally or feet vertically
• Fuel Planning: Endurance calculated as fuel available ÷ fuel burn per NM

Universal Standard: ICAO standards mandate nautical miles globally. Even countries using metric on land (Europe, Asia) use NM in aviation.

3. Military Operations and Defense

Naval and air forces worldwide use nautical miles:

• Tactical Planning: Mission ranges, patrol areas, weapon ranges all in NM
• Rules of Engagement: May specify engagement zones as X NM from assets
• International Waters: Freedom of navigation operations occur beyond 12 NM territorial limit
• Exercise Areas: Military training areas defined by coordinates with dimensions in NM

Interoperability: NATO and allied forces must use common units—nautical miles ensure coordination.

4. Oceanography and Marine Science

Scientists studying oceans use nautical miles naturally:

• Research Vessel Cruises: Tracks measured in nautical miles sailed
• Acoustic Surveys: Transects for fish surveys measured in NM
• Ocean Currents: Velocities in knots, distances in NM
• Whale Migration: Tracked in nautical miles traveled per day

Coordinate Integration: Scientific data tagged with lat/lon coordinates fits naturally with nautical mile distances.

5. Maritime Law Enforcement and Border Control

Coast guards and maritime police use nautical miles:

• Patrol Areas: Assigned patrol zones measured in square NM
• Pursuit Distances: Hot pursuit laws reference territorial limits (12 NM)
• Smuggling Interdiction: Intercept calculations based on target speed (knots) and distance (NM)
• Fisheries Enforcement: EEZ boundaries (200 NM) patrol and enforcement

6. Marine Charts and Navigation Publications

All official charts use nautical miles:

• Paper Charts: Latitude scale serves as distance ruler (1 minute = 1 NM)
• Electronic Charts (ECDIS): Display distances in NM by default
• Sailing Directions: Describe routes, distances, hazards using NM
• Light Lists: Lighthouse visibility ranges listed in nautical miles

Chart Scales: Often expressed as 1:X where X determines detail level. Common scales like 1:50,000 mean 1 cm on chart = 0.5 km = ~0.27 NM.

7. Weather Routing and Voyage Optimization

Modern shipping uses weather forecasting to optimize routes:

• Weather Routing Services: Calculate optimal track to minimize voyage time and fuel
• Forecast Models: Wind/wave forecasts presented with positions in lat/lon and coverage in NM
• Routing Algorithms: Evaluate alternatives by comparing total NM distance + weather impacts
• Fuel Savings: Avoiding storms may add 50 NM but save days and tons of fuel

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How to Convert Nautical Miles to Other Length Units

Basic Conversion Formulas

1 nautical mile = 1,852 meters (exactly)
1 nautical mile = 1.852 kilometers
1 nautical mile = 6,076.115 feet
1 nautical mile = 2,025.372 yards
1 nautical mile = 1.15078 statute miles

Geographic:
1 nautical mile = 1 minute of arc (latitude)
60 nautical miles = 1 degree of latitude

Nautical Miles ↔ Other Length Units

From	To	Multiply by
Nautical Miles	Meters	× 1,852
Nautical Miles	Kilometers	× 1.852
Nautical Miles	Feet	× 6,076.115
Nautical Miles	Statute Miles	× 1.15078
Meters	Nautical Miles	÷ 1,852
Kilometers	Nautical Miles	÷ 1.852
Statute Miles	Nautical Miles	÷ 1.15078 (or × 0.86898)

Speed Conversions (Knots ↔ Other Speed Units)

From	To	Multiply by
Knots (NM/h)	Kilometers/hour	× 1.852
Knots	Miles/hour (mph)	× 1.15078
Knots	Meters/second	× 0.51444
km/h	Knots	÷ 1.852
mph	Knots	÷ 1.15078

Examples

Example 1: Ship Voyage Distance

• Distance from San Francisco to Honolulu: 2,100 nautical miles
• Convert to kilometers: 2,100 × 1.852 = 3,889.2 km
• Convert to statute miles: 2,100 × 1.15078 = 2,416.6 statute miles

Example 2: Hurricane Wind Speed

• Hurricane Category 3: Winds of 100 knots
• Convert to km/h: 100 × 1.852 = 185.2 km/h
• Convert to mph: 100 × 1.15078 = 115.1 mph

Example 3: Aircraft Speed

• Boeing 737 cruises at 450 knots
• Convert to km/h: 450 × 1.852 = 833.4 km/h
• Convert to mph: 450 × 1.15078 = 517.9 mph

Example 4: Latitude to Distance

• You sail from 30°N to 35°N latitude
• Latitude change: 5 degrees × 60 = 300 nautical miles (exactly)
• In kilometers: 300 × 1.852 = 555.6 km

Example 5: Territorial Waters

• 12 nautical mile territorial limit
• In kilometers: 12 × 1.852 = 22.224 km (often rounded to 22.2 km)
• In statute miles: 12 × 1.15078 = 13.81 statute miles

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Common Conversion Mistakes to Avoid

1. ❌ Confusing Nautical Miles with Statute Miles

Mistake: "The ship is 100 miles offshore" (ambiguous—nautical or statute miles?)

Problem: Nautical mile is 15% longer than statute mile. Using wrong unit causes significant errors.

✅ Correct: Always specify: "100 nautical miles offshore" or "115 statute miles offshore."

Practical Impact:

• 100 NM = 115 statute miles = 185 km
• Assuming statute miles when distance is in NM: 15% error
• In navigation, 15% error can mean missing destination by many miles

2. ❌ Assuming Latitude and Longitude Minutes Are Equal Distance

Mistake: "1 minute of longitude = 1 nautical mile everywhere on Earth."

Problem: Longitude lines converge at poles. 1 minute of longitude = 1 NM only at the equator.

✅ Correct:

• Latitude: 1 minute = 1 nautical mile everywhere (along meridians)
• Longitude: 1 minute = 1 NM × cos(latitude)
  • At equator (0°): 1 min longitude = 1 NM
  • At 45°N: 1 min longitude = 0.707 NM
  • At 60°N: 1 min longitude = 0.5 NM
  • At 89°N: 1 min longitude = ~0.017 NM

Navigation Tip: Always measure distances using the latitude scale on nautical charts, not the longitude scale.

3. ❌ Using Imprecise Conversion Factors

Mistake: "1 nautical mile = 1.85 km" (rounded excessively)

Problem: Over long distances, rounding errors accumulate.

✅ Correct: Use 1,852 meters (exact) or 1.852 km (exact to three decimals).

Error Example:

• Actual: 1,000 NM = 1,852 km
• Using 1.85 km: 1,000 NM = 1,850 km
• Error: 2 km (not huge, but unnecessary inaccuracy)

4. ❌ Mixing Knots and Miles Per Hour

Mistake: "The ship is traveling at 20 miles per hour" (when speed is actually 20 knots).

Problem: 20 knots = 23 mph. Using wrong unit underestimates speed by 15%.

✅ Correct:

• Maritime/aviation: Always use knots
• Land vehicles: Use mph or km/h
• Converting: 20 knots × 1.15078 = 23.0 mph

Red Flag: If you hear "miles per hour" in maritime/aviation context, clarify whether they mean knots.

5. ❌ Forgetting That Nautical Mile Is Defined in Meters, Not Feet

Mistake: "The nautical mile is exactly 6,076 feet."

Problem: 6,076 feet is approximate. The official definition is 1,852 meters (exact).

✅ Correct:

• 1 NM = 1,852 m (exact, by international definition since 1929)
• 1 NM = 6,076.115 feet (derived from meter definition)

Why It Matters: In legal/regulatory contexts, the meter-based definition is authoritative.

6. ❌ Assuming "Mile" Means Statute Mile in Maritime Contexts

Mistake: Reading a maritime report: "Ship is 50 miles from port" and assuming statute miles.

Problem: In maritime contexts, "mile" always means nautical mile unless explicitly stated otherwise.

✅ Correct:

• Maritime/aviation: "Mile" = nautical mile (default)
• Land/road context: "Mile" = statute mile (default)
• To avoid confusion: Specify "nautical mile" or "statute mile" when ambiguity possible

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