Qoppa
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Qoppa (Ϙ or ϟ) was a letter of the Greek alphabet. It is no longer used in normal writing today. It stood for the sound "[q]" (similar to [k]) in Ancient Greek. In the order of the letters in the alphabet, it came between Pi and Rho, just like in the Latin alphabet the letter Q comes between P and R. Like all Greek letters, it could also be used to write a number, and then it stood for the number 90. People still sometimes use it in this way, even though they don't use it as a letter to write words.
The letter qoppa at first looked like a circle with a small bar under it. Later it was written more like a pointed S.
The letter Q in the Latin alphabet and the letter Ҁ in the Cyrillic alphabet come from Qoppa.