Classical Greek does not use a separate letter for the “h”, or aspirated breathing sound. You will recall, for example, that the aspirated versions of π, τ, and κ are represented by different letter forms: φ, θ, and χ.
In some cases, the “h” breathing sound was also applied to the BEGINNING of several Greek words, all of which begin with a vowel, diphthong, or the letter ρ. For words that begin with a vowel or diphthong, there are two symbols that indicate the presence or lack of aspiration: a SMOOTH BREATHING mark indicates no aspiration, and a ROUGH BREATHING mark indicates aspiration (S 9-14, G 6).
In the case of the consonant ρ, it always receives ROUGH BREATHING when found at the BEGINNING of a word. This phenomenon is reflected in the spelling of English words of Greek origin, such as “rhetoric” and “rhythm.” If ρ is found within or at the end of a word, no breathing mark is applied.
᾽ e.g., ὀ = “o” smooth breathing
῾ e.g., ὁ = “ho” rough breathing
For single vowels and the consonant ρ, the marks are placed directly above the lower-case letters (e.g. ὁ and ῥ), and immediately to the left of capital letters (e.g. Ὁ and Ῥ). If placed over a DIPHTHONG, the mark is always above the SECOND vowel (e.g. οἱ, εἰ, and Εὐρώπη).
