Source : Webster's Revised Unabridged Dictionary (1913)

R \R\ ([aum]r).
   R, the eighteenth letter of English alphabet, is a vocal
   consonant. It is sometimes called a semivowel, and a liquid.
   See Guide to Pronunciation, [sect][sect] 178, 179, and
   250-254. ``R is the dog's letter and hurreth in the sound.''
   --B. Jonson.

   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 derived from
         the Ph[oe]nician, 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.