r /ˈɑr/
 字母r
 R R, the eighteenth letter of the English alphabet, is a vocal consonant.  It is sometimes called a semivowel, and a liquid. See Guide to Pronunciation, §§ 178, 179, and 250-254. “R is the dog's letter and hurreth in the sound.”
 Note: In words derived from the Greek language the letter h is generally written after r to represent the aspirated sound of the Greek "r, but does not affect the pronunciation of the English word, as rhapsody, rhetoric.
     The English letter derives its form from the Greek through the Latin, the Greek letter being derived from the Phœnician, which, it is believed, is ultimately of Egyptian origin. Etymologically, R is most closely related to l, s, and n; as in bandore, mandole; purple, L. purpura; E. chapter, F. chapitre, L. capitulum; E. was, were; hare, G. hase; E. order, F. ordre, L. ordo, ordinis; E. coffer, coffin.
 The three Rs, a jocose expression for reading, (w)riting, and (a)rithmetic, -- the fundamentals of an education.
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 R
      n 1: a unit of radiation exposure; the dose of ionizing radiation
           that will produce 1 electrostatic unit of electricity in
           1 cc of dry air [syn: roentgen]
      2: (physics) the universal constant in the gas equation:
         pressure times volume = R times temperature; equal to
         8.3143 joules per kelvin per mole [syn: gas constant, universal
         gas constant]
      3: the 18th letter of the Roman alphabet
      4: the length of a line segment between the center and
         circumference of a circle or sphere [syn: radius]