Why are there two lower case sigmas in the Greek alphabet?

Answer 1

Συμφωνώ με το συνάδελφο,

You use -σ in any case you need a non capital /s/ and -ς τελικό ( Teliko) only at the ending is a word. That’s why that sigma is called teliko which means last in Greek.

examples:

Νεανίας

ανδρας

ανθρωπος

συνεση

συνεδριασης

συνδραμεις

Do it make sense?

Answer 2

σ is the standard lowercase sigma and ς is only used at the very end of a word.