He is best known for his Account of Letters ('О писменех / o pismenex /), a chronicle of the development of Cyrillic. He wrote of the situation of the Slavs before the invention of Cyrillic and described the impact of the new script:
Being still pagans, the Slavs did not have their own letters, but read and communicated by means of tallies and sketches. After their baptism they were forced to use Roman and Greek letters in the transcription of their Slavic words but these were not suitable ... At last, God, in his love for mankind, sent them St. Constantine the Philosopher, called Cyril, a learned and upright man, who composed for them thirty-eight letters, some (24 of them) similar to the Greek, but some (14 of them) different, suitable to express Slavic sounds.
In doing so, he provided information critical to Slavonic palaeography and records that the pre-Christian Slavs employed a "stroke and incision" (черты и резы /chert1 i rez1/, translated as "tallies and sketches" above) writing that was, apparently, insufficient properly to reflect the spoken language. It is thought that this may have been a form of runic script but no authentic examples are known to have survived.
But most probably the brothers created the highly elaborated Glagolitic alphabet, while their disciples in Bulgaria, most outstanding among whom was St. Kliment of Ohrid, created the Cyrillic alphabet, naming it after their teacher St. Cyril.
The significance of the new letters was highly praised, odes were written in celebration of this historical deed, which was cherished not simply as patriotic, but mainly in terms of "untying the tongue and teaching reason", as Chernorizetz Khrabr puts it in his "Account of Letters".
The Bible and lots of other Christian books were first translated into Bulgarian, and new books were written in the old Bulgarian language.