R, a liquid consonant, being the eighteenth letter of the English alphabet. Its sound is formed by a guttural extrusion of breath vibrated through the mouth, with a sort of quivering motion of the tongue drawn back from the teeth, and advanced with the tip a little elevated towards the palate. It is convertible with the letters l, n, m, and s. It is apt to place itself at one time before, at another time after a vowel, and is liable to disappear from the neighbourhood of several consonants.

In the notes of the ancients, R. or RO. signifies Roma. R. C. Romana civitas; R. G. C. rei gerenda causa; R. F. E. D. recte factum et dictum; R. G. F. regis filius; R. P. res publica, or Romani principes, and R. R. R. F. F. F. res Romana ruet ferro, fame, flamma. (See ABBREVIATIONS.) Used as a numeral, R. anciently stood for 80; and with a dash over it, thus \text{R}^{\text{—}}, for 80,000; but the Greek R, \rho, with a small mark over it, signified 100; with the same mark under it, it denoted 1000 \times 10; thus \rho, signifies 100,000. In the Hebrew numeration \text{ר} denoted 200; and with two horizontal points over it, thus \text{ר}^{\text{—}}, 1000 \times 200; = 200,000. In the prescriptions of physicians, R or \text{R}^{\text{—}} stands for recipe, that is, "take."