Pi-mutes π β φ ψ
Tau-mutes τ δ ϑ ζ

§ 14. A _smooth_ final mute is roughened before a vowel with the
rough breathing. A rough mute is not doubled, nor can successive
syllables begin with an aspirate. A tau-mute is sometimes dropped
before σ, and always before κ; before a different tau-mute it is
changed into σ.

§ 15. Before μ a kappa-mute is changed into γ, a pi-mute into μ,
and a tau-mute into σ.

§ 16. ν before a kappa-mute becomes γ, before a pi-mute μ, before
a liquid (λ, μ, ν, or ρ) it is changed into the same liquid,
before σ or ζ it is dropped.

§ 17. ν is appended to certain endings in σι or ε before a pause
or a vowel.

§ 18. There are several other less important rules, and some
exceptions to most of the above.

§ 19. A _long vowel_ or _diphthong_ is used as an
equivalent for two (usually short) vowels in immediate succession, or
as a compensation for the omission of a consonant, sometimes for both.

§ 20. The changes in the union of two vowels are various, depending
upon their comparative strength, position, and relation to the long
vowels, or diphthongs respectively. They are readily learned by
practice.