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'Er' (Р, р) is the eighteenth letter of the Cyrillic alphabet. It was developed from the Greek letter Rho. Er looks exactly the same as the upper case Rho. Er also looks exactly the same as the Latin letter P.
It basically corresponds to the Latin letter R and represents , an alveolar trill. When followed by a palatalizing vowel, it represents .
The drawing of 'R' differs in that it has a line attached to it, which was given by the Romans to distinguish it from the letter P, which is not at all related.
Russian Er is always transliterated into the Latin alphabet as 'R'.