Most greek consonants can be divided into three classes, labial, dental and palatal. Each of these classes has plain, voiced, aspirated or fricative and a double consonant (doubled with σ) sounds. The following chart should illustrate the relationships between consonants:
|Plain||Voiced||Aspirated||Doubled with σ|
For each row, any plain, voiced or aspirated sound combined with σ yields the doubled consonant. Originally, the aspirated consonants represented the plain sound plus a puff of air. Because of this relationship, when a plain sound is followed immediately by a rough breathing, it is written and pronounced aspirated. (i.e. -π’ + ἁ- yields -φ’ ἁ-)
This leaves the consonants ρ, λ, μ, ν and σ (and the rough breathing “h-“) which do not fall into the labial/dental/palatal classification system.