R, 18th letter of the alphabet, corresponding to Greek rho. When in Latin alphabets the letters for p and r became similar in appearance, the rho form (P; which at first was used for the r sound) was restricted to the p sound, and an extra line was added for the r sound (R). In Greek, where no confusion developed, the rho continued to look like Latin P. A modification of R is the symbol ℞, used medically for recipe [Lat., = take] and liturgically for responsum [Lat., = response].
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