Koppa or Qoppa is a letter that was used in early forms of the Greek alphabet

Greek alphabet

The Greek alphabet is the script that has been used to write the Greek language since at least 730 BC . The alphabet in its classical and modern form consists of 24 letters ordered in sequence from alpha to omega...

, derived from Phoenician

Phoenician alphabet

The Phoenician alphabet, called by convention the Proto-Canaanite alphabet for inscriptions older than around 1050 BC, was a non-pictographic consonantal alphabet, or abjad. It was used for the writing of Phoenician, a Northern Semitic language, used by the civilization of Phoenicia...

 qoph

Qoph

Qoph or Qop is the nineteenth letter in many Semitic abjads, including Phoenician, Aramaic, Syriac, Hebrew and Arabic alphabet . Its sound value is an emphatic or . The OHED gives the letter Qoph a transliteration value of Q or a K and a final transliteration value as a ck...

. It was originally used to denote the /k/ sound, but dropped out of use as an alphabetic character in favour of Kappa (Κ). It has remained in use as a numeral symbol (90) in the system of Greek numerals, although with a modified shape. Koppa is the source of Latin

Latin alphabet

The Latin alphabet, also called the Roman alphabet, is the most recognized alphabet used in the world today. It evolved from a western variety of the Greek alphabet called the Cumaean alphabet, which was adopted and modified by the Etruscans who ruled early Rome...

 Q

Q

Q is the seventeenth letter of the basic modern Latin alphabet.- History :The Semitic sound value of Qôp was , a sound common to Semitic languages, but not found in English or most Indo-European ones...

, as well as the Cyrillic

Cyrillic alphabet

The Cyrillic script or azbuka is an alphabetic writing system developed in the First Bulgarian Empire during the 10th century AD at the Preslav Literary School...

 numeral sign of the same name (Koppa).
In Phoenician, qoph was pronounced q; in Greek, which lacked such a sound, it was instead used for /k/ before back vowel

Back vowel

A back vowel is a type of vowel sound used in spoken languages. The defining characteristic of a back vowel is that the tongue is positioned as far back as possible in the mouth without creating a constriction that would be classified as a consonant. Back vowels are sometimes also called dark...

s, Ο, Υ and Ω.