Continuing with the sound system of classical Greek, we find an aspirated sound, read “h-“, made by an extra puff of air. Rather than indicating with another character, Greek employs a breathing mark (ʽ), called a rough breathing. It is placed above the first character in the word (or before the character, in the case of upper case letters).
Not all words that begin with a vowel require this rough breathing, of course. Those beginning with a vowel and not requiring aspiration always take a smooth breathing mark (ʼ). It is placed in the same way as the rough breathing mark.
As a rule, words beginning with upsilon (υ) always take a rough breathing. In addition, the character rho (ρ), when placed at the beginning of a word, always takes a rough breathing. This breathing is not actually pronounced. Rho is the only consonant that ever receives a breathing mark.
Rough breathing: ὑπό, hupo (a preposition)
Smooth breathing: ἔργον, ergon (a work or deed)