Henry George Liddell, Robert Scott, An Intermediate Greek-English Lexicon

Ζ, ζ, ζῆτα, τό, indecl., sixth letter of Gk. Alphabet: as numeral ζ# ῀ ἑπτά and ἕβδομος (the obsol. `ς`#, i. e. ϝ, vau, the digamma, being retained to represent ἕξ, ἕκτος) , but #22ζ = 7000. Ζ ζ is composed of ς and δ, so that in aeolic it becomes σδ, as Σδεύς κωμάσδω ψιθυρίσδω for Ζεύς κωμάζω ψιθυρίζω reversely, in attic, σδ becomes ζ, Ἀθήναζε θύραζε for Ἀθήνασδε θύρασδε. But ς often disappears in aeolic, where ζά ῀ δια, see ζά, ζα-so in aeolic and doric, as we have Δεύς Δάν for Ζεύς Ζάν, δορκάς ῀ ζορκάς:—so also ἀρίζηλος for ἀρίδηλος; ἀλαπαδνός from ἀλαπάζω, παιδνός from παίζω:—doric, in the middle of words, it becomes δδ, as θερίδδω for -ίζω, μάδδα for μᾶζα. Zeta, being a double conson., made a short vowel at the end of the foregoing syllable long by position. But Homer used the vowel short before two prop. names, which could not otherwise come into the Hexam., viz. ἄστυ^ Ζελείης, ὑλήεσσα^ Ζάκυνθος.