The major constituent tribes or tribal groupings of the Qays were the Hawazin, Banu 'Amir, Banu Thaqif, Banu Sulaym, Banu Ghani, Bahila and Banu Muharib. Many of these tribes or their clans migrated from the Arabian Peninsula and established themselves in northern Syria and Upper Mesopotamia, which long became their abode. From there they governed on behalf of the caliphs or rebelled against them. The power of Qays as a unified group diminished with the rise of the Abbasid Caliphate which did not derive its military strength solely from the Arab tribes. Nonetheless, individual Qaysi tribes remained a potent force and some migrated to North Africa and Iberia where they carved out their own power.
The full name of the tribal confederation is Qays ʿAylān or Qays ibn ʿAylān, though it is most frequently referred to simply as Qays; occasionally in Arabic poetry, it is referred to solely as ʿAylān. Members of the Qays are referred to as al-Qaysĭyūn (sing. Qaysī), transliterated in English-language sources as "Qaysites" or "Kaisites". As an ethno-political group, the Qays are referred to in contemporary sources as al-Qaysīyya. Unlike most Arab tribes, the sources seldom use the term Banū (sons of) when referring to the descendants of Qays.
Qays is the namesake and progenitor of the confederation, and traditional Arab genealogy holds that the father of Qays was a certain 'Aylan. According to the genealogists, "'Aylan" was actually the epithet of al-Nās, a son of Mudar ibn Nizar ibn Ma'ad ibn Adnan. The theory that Aylan is the father of Qays is rejected by Ibn Khaldun (d. 1406), a medieval historian of Arab tribes, and is indirectly rejected by other medieval Arab historians. Rather, Ibn Khaldun asserts that "Qays 'Aylan" is the epithet of al-Nas ibn Mudar ibn Nizar ibn Ma'ad ibn Adnan. These historians hold varying theories as to the origins of the "Aylan" part of the epithet; among these are that 'Aylan was either the name of al-Nas's famous horse, his dog, his bow, a mountain where he was said to have been born, or a man who raised him.
Qays was one of the two subdivisions of Mudar, the other being Khindif (also known as al-Yās). As descendants of Mudar, the Qays are considered Adnanites or "North Arabians"; Arab tradition traces the descent of all Arab tribes to either Adnan or Qahtan, father of the "South Arabians". By the dawn of Islam in the mid-7th century, the descendants of Qays were so numerous and so significant a group that the term Qaysī came to refer all North Arabians.