Note: Do not rely on this information. It is very old.
RR, the eighteenth letter of the English and the seventeenth of the Latin alphabet, derived through the Greek p from the Phoenician Resh. It is classed as a semi-vowel or liquid. In most languages, and even in Central and Southern English, before vowels it is'a trill or broken murmur produced by raising the tip of the tongue up close to the palate, and making it vibrate by an emission of voiced breath and then lowering the tongue-tip. There are several varieties of this sound, of which the ordinary English variety is the weakest, being either a slight murmur or quite suppressed before consonants, or when final, as in art, ever; but in these positions, and sometimes also when followed by mute e, the preceding vowel is modified, as in pert, first, curd, word, horse, per, fir, fur, for, here, pure, sore, care, while a before r and a consonant, or accented before final r, retains its old open sound, and ar = d, as in art, harm, farce, guitar. The r grasseye is formed by the vibration of the uvula and velum pendulum. The same sound is given to rr as to r; e.g. carrot, carol, ferry, very, florid, torrid, card, charred. In several, languages - e.g. in English and Latin - r has succeeded a voiced sibilant (English z), as in hare for Teutonic haza.