R /

R ( /ˈɑr/, Hiberno-English: /ˈɔr/; named ar or or) is the eighteenth letter of the basic modern Latin alphabet. The original Semitic letter was probably inspired by an Egyptian hieroglyph for tp, "head". It was used for /r/ by Semites because in their language, the word for "head" was rêš (also the name of the letter). It developed into Greek Ρ ῥῶ (rhô) and Latin R. It is likely that some Etruscan and Western Greek forms of the letter added the extra stroke to distinguish it from a later form of the letter P. The minuscule (lower-case) form of r developed through several variations on the capital form. In handwriting it was common not to close the bottom of the loop but continue into the leg, saving an extra pen stroke. The loop-leg stroke shortened into the simple arc used today. Another minuscule, r rotunda (ꝛ), kept the loop-leg stroke but dropped the vertical stroke, although it fell out of use around the 18th century. In science, the letter R is a symbol for the gas constant. Mathematicians use R or (an R in blackboard bold, displayed as ℝ in Unicode) for set of all real numbers. R represents a rhotic consonant in many languages, as shown in the table below.