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
Labial	π	β	φ	ψ
Dental	τ	δ	θ	ζ
Palatal	κ	γ	χ	ξ

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.