The Collaborative International Dictionary of English v.0.48:

R \R\ ([aum]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, [sect][sect] 178, 179, and
   250-254. "R is the dog's letter and hurreth in the sound."
   --B. Jonson.
   [1913 Webster]

   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.
         [1913 Webster] The English letter derives its form from
         the Greek through the Latin, the Greek letter being
         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.
         [1913 Webster]

   The three Rs, a jocose expression for reading, (w)riting,
      and (a)rithmetic, -- the fundamentals of an education.
      [1913 Webster]