The Quran is preserved not in one single reading but in a small number of canonical variant reading traditions, qira'at, differing in matters such as vowelling, pronunciation and occasionally word choice while conveying the same consonantal text, and the tenth century scholar Ibn Mujahid is credited with formally fixing seven of these as canonically authoritative, a number later traditions expanded to ten; a qari trains in one or more of these named reading traditions, and which reading is used in a given country today generally reflects a specific historical transmission rather than any live disagreement about the text's authenticity.
Radio broadcasting transformed the reach of the office in the twentieth century, turning a small number of Egyptian reciters, most famously Abdul Basit Abdul Samad, into figures whose recitations were heard across the entire Muslim world via state radio, cassette tape and, more recently, satellite television and internet streaming, so that a listener in almost any country can now access recitations by qaris whose voices, a century earlier, would have been audible only within a single mosque or a single city.
Formal certification follows the same chain-of-transmission model used across other branches of Islamic scholarship: a qari receives an ijazah, a written certificate naming the specific teacher and reading tradition studied, only after reciting the entire Quran correctly before that teacher from memory or from the text, and a qari who wishes to teach in turn must hold such a certificate reaching back through named teachers to an early authority, so that the credential is not a general diploma but a documented personal chain a student can trace teacher by teacher.