They manuals are parts of the Shiksha Vedanga: works dealing with the phonetic aspects of the Sanskrit language used in the Vedas. Each Veda has a "pratishakhya" for each school. Many "pratishakhyas" have survived into the modern age, and, according to Hartmut Scharfe, all except one ("Taittiriya pratisakhya") are based upon "recitation of isolated words". Pratishakhyas begin with word-for-word recitation, and then supply rules for the continuous recitation of texts. Though all the manuals have the same basic goal, they differ significantly in how each achieves that goal. They were composed centuries before the work of Pāṇini, but there is evidence in these manuals that many "pratisakhya" evolved and were revised over time by the respective school to their regional preferences. The few manuscripts of the "pratisakhyas" that have survived into the modern era are likely from the 500 to 150 BCE period. The phonetic aspects of "Vajasaneyi Pratisakhya" is closest to those found in the classic Sanskrit grammar work of Pāṇini.