# R

**R** is the eighteenth letter of the Latin alphabet.

Semitic Rêš (the head) developed into Greek Ρω (Rô). The sound value /r/ however was maintained in Greek as well as Etruscan and Latin.

**Romeo** represents the letter R in the NATO phonetic alphabet.

## Meanings for R

- In biochemistry, R is the symbol for arginine.
- In calendars, R is often used as an abbreviation for Thursday instead of T, to avoid confusion with Tuesday.
- In chemistry,
- R denotes the molar gas constant.
- In organic chemistry, R is the symbol for the hydrocarbon chain.

- in chess, R is a notation symbol for the rook piece
- In commerce, R with an enclosing circle, ®, denotes a registered trademark.
- In computing, R is the name of a statistical programming language; see R (programming language)
- In electrical engineering, R is often the variable for electrical resistance
- In film, R is a rating given for films given by the Motion Picture Association of America; see R (rating)
- In financial securities, R is the stock symbol for Ryder Systems Inc
- In geometry, r is often a variable for the radius of a circle
- In law, R stands for
*Rex*(king) or*Regina*(queen) in court citations incountries that recognize the British monarch as head of state. - In mathematics, blackboard bold represents the real numbers
- In navigation, R denotes the relative direction "right"; see left and right.
- In physics, R is the symbol for roentgen, a unit of dosage of X-rays or gamma radiation.
- In radiocommunication, R is one of the ITU prefixes allocated to Russia.

Two-letter combinations starting with R: