They were percussion instruments, (known since the Minoan culture) which were held and struck rhythmically with the hands, usually accompanying orgiastic rituals (of Cybele and Dionysus).
They consisted of two concave hemispherical discs (5 to 18 cm in diametre) of hammered cast bronze.
The smallest had a leather loop in their centre for a secure hold of the player’s thumb and middle finger. The larger ones had bronze rings in the centre (through which the fingers could be inserted) connected with leather straps or chain.

ΠΗΓΗ: Μ.Λ. Γουέστ, Αρχαία ελληνική μουσική, Σόλων Μιχαηλίδης, Εγκυκλοπαίδεια της αρχαίας ελληνικής μουσικής, Πολυδεύκης, Ονομαστικόν

SOURCES: M.L. West, Ancient Greek Music, Solon Michailidis, Encyclopaedia of Ancient Greek Music, Pollux, Onomasticon.