It is the eighth letter of the Greek alphabet, and is known to have been derived from the Phoenician letter Teth. Both resemble the number 0 or a circle or oval with a horizontal line in the middle. Theta, written in ancient Greek as θῆτα and as θήτα in modern Greek has an uppercase (Θ or ϴ) and lowercase (θ) symbol.
3.3 How to Type Theta Symbol on Mac Theta – Origins and Symbolisms
