The Greek character set has been designed with the common GKOS goal in mind that native versions should be compatible phonetically with each other as much as possible. This way the typing skill obtained can be used effectively when switching between layouts. As letters C and W do not exist in Greek, and B (as in 'best', typed 'μ' followed by 'π') is somewhat rare in Greek, Keys [B] and [C] are used for letters o-micron and ι that are frequent in Greek. Now, o-micron (O/ο) and o-mega (Ω/ω) reside partly in the same Keys (in a logical way); so do also sigma (Σ/σ), word-final sigma (ς) and Z/ζ. Iota (Ι/ι) is in the same Key position as [J] but without using Modifier [G]. Acute accent is very common and it has been assigned the 'light-weight' Combo [W].
Note:
Letters in Modern Greek in an approximate order of frequency: οαιρτσςμνευλπηκγχφδωθβξζψ