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R, or r, is the eighteenth letter of the English alphabet.

R or r may also refer to:

Science

Biology and medicine

• Arginine, an amino acid abbreviated as Arg or R
• ATC code R Respiratory system, a section of the Anatomical Therapeutic Chemical Classification System
• Coefficient of relationship (r), in biology
• Effective reproduction number (R), the number of cases generated by one case in the current state of a population in epidemiology
  • Basic reproduction number (R₀), the expected number of cases directly generated by one case
• Haplogroup R (mtDNA), a human mitochondrial DNA (mtDNA) haplogroup
• Haplogroup R (Y-DNA), a Y-chromosomal DNA (Y-DNA) haplogroup
• Net reproduction rate (R₀), the average number of offspring that would be born to a female given conforming conditions
• r, the population growth rate in the r/K selection theory of ecology

Astronomy

• Spectral resolution (${\displaystyle \mathbb {R} }$), in astronomy
• Orangish or K carbon stars (stellar classification: R)

Physics

• R (cross section ratio), the ratio of hadronic to muonic cross sections
• Electrical resistance (R)
• Roentgen (unit) (R), a unit of measurement for ionizing radiation such as X-ray and gamma rays
• Rydberg constant (R_∞, R_H), a physical constant relating to energy levels of electrons within atoms
  • Rydberg unit of energy (R_y), the energy of the photon whose wavenumber is the Rydberg constant

Temperature scales

• Rankine scale (°R, °Ra)
• Réaumur scale (°Ré, °Re, °r)
• Rømer scale (°R, °Rø)

Chemistry

• Gas constant (R), in chemistry
• R, one of two chiral center configurations in the R/S system
• Alkyl group, commonly marked as R in structural formulas
• Side chain (an unspecified group of atoms), in a chemical or structural formula

Mathematics

• r is the symbol for the radius of a circle
• ${\displaystyle \mathbb {R} }$ or R, the set of all real numbers
• Pearson product-moment correlation coefficient (r), in statistics
• R (programming language), an environment for statistical computing and graphics
• Ricci curvature
• Γ, the gamma function

Technology and engineering

• R-value (insulation), a non-SI measure of thermal resistance used in housing insulation
• R-value (soils), property of soils used in pavement design
• Resistor, an electronic component
• R-, a refrigerant numbering system
• R, the ratio between the minimum and maximum stresses in fracture mechanics
• R- or recall-button, now mostly used to switch between two calls

Computing

• R (programming language), an environment for statistical computing and graphics
• R (complexity), the set of all recursive languages
• IBM System R, an IBM database system
• RequestBot, a service in QuakeNet's IRC services

Arts and entertainment

Music

• Ritardando, a term for slowing the tempo down gradually
• R. (R. Kelly album), a 1998 album by R. Kelly
• R° (Ruratia album), a 2002 album by Rurutia
• R (single album), a 2021 single album by Rosé
• Rated R (Queens of the Stone Age album), 2000 album also called R
• R (jp), 2005 J-Pop album by Ryoko Shiraishi

Film

• R (film), a 2010 film by Danish screenwriter and film director Tobias Lindholm
• R, a character played by John Cleese in The World Is Not Enough
• R, the protagonist of Warm Bodies, based on the book of the same name
• R, a film rating in the Motion Picture Association of America film rating system and in the Canadian Home Video Rating System

Television

• Rajawali Televisi (RTV), television in Indonesia
• R, the production code for the 1965 Doctor Who serial The Chase

Video games

• R·Type, an arcade video game
• R: Rock'n Riders, in the list of PlayStation games (M–Z)

Organisations

• The Crown, Rex (King) or Regina (Queen) in Commonwealth nations
• Ryder (NYSE ticker symbol)
• R (cable operator), a telecommunications company
• R, denoting a member of the US Republican Party

Transportation

• Nissan Skyline GT-R, commonly referred to as the "R" among Japanese speakers
• R (New York City Subway service)
• Rinkai Line, a railway line operated by the Tokyo Waterfront Area Rapid Transit, labeled R
• Transilien Line R, a line of the Paris transport network
• Reconnaissance aircraft (military designation: R)
• Restricted airspace
• Volkswagen R, the performance division of Volkswagen
• The official West Japan Railway Company service symbol for:
  • Hanwa Line.
  • San'yō Main Line.

Other uses

• Registered trademark symbol (®)
• South African rand (R), a currency
• Right (direction)
• Rolled R (IPA: r), the alveolar trill of Spanish, Italian, & Arabic languages
• UTC−05:00 (military time zone code: Romeo)
• Я, a Cyrillic letter similar to R
• Subreddit, user-created areas of interest where discussions on Reddit are organized.

See also

• R. (disambiguation)
• "R" Is for Ricochet, the eighteenth novel in Sue Grafton's "Alphabet mystery" series, published in 2004
• The three Rs, of reading, writing and arithmetic
• RR (disambiguation)
• RRR (disambiguation)
• RRRR (disambiguation)
• R class (disambiguation)
• Model R (disambiguation)
• Type R (disambiguation)

• Disambiguation pages