Webster's Revised Unabridged Dictionary · 1913

R

R (ar)

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. — B. Jonson
Collocations (1)
The three Rs , a jocose expression for reading, (w)riting, and (a)rithmetic, -- the fundamentals of an education.