by George Valsamis
Groups of vowels and consonants
Here are the Greek letters divided in vowels of long and/or short duration
and 7 kinds of consonants:
Can be long or short:
Αα, Ιι, Υυ
Κκ, Ππ, Ττ
Χχ, Φφ, Θθ
Ββ, Γγ, Δδ
Ζζ, Ξξ, Ψψ
Memorize this list, especially the duration of the vowels, because you will need it later to understand the use of accentuation marks.
Note: Double consonants (ζ, ξ, ψ) are named so, because they were formed by the absorption of two letters. You can see below their origin, not always as obvious as e.g. in π + σ giving ψ.
ζ comes from σ+δ (Ἀθήνασδε -> Ἀθήναζε) or from δ+j
ξ comes from κ+σ, or γ+σ, or χ+σ
ψ comes from π+σ, or β+σ, or φ+σ.
Reference address : https://www.ellopos.net/elpenor/lessons/lesson1.asp?pg=6