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