When a Greek word begins either with a vowel or a rho, the first letter must have a breathing mark above it. There are two breathing marks, one that looks like (, and one that looks like ). The ( (rough breathing) simply indicates that you must pronounce an h sound before the word, the ) (smooth breathing) indicates that there is no h.

A rho will always have rough breathing, without exception. This is why English words which derive from Greek words which begin with rho are spelled in English beginning with rh.