The manuscript fragment in the collection of the National Museum Library in Prague under shelf mark 1 D a 3/52 is a sheet of paper with writing on both sides, containing two strata of inscriptions. The first stratum consists of accounting records, one of which is dated to 1356. That is also the terminus post quem for the other stratum of inscriptions, namely the musical notation of two liturgical plainchants in two-voice organ paraphrases. This involves the introit Salve, sancta parens and the Kyrie magne Deus. The discant is written in black mensural notation on a staff, while the tenor, which quotes the plainchant melody, is partially written in musical notation on the same staff, partially notated by letters for note names, and partially only indicated by syllables of text of the original plainchant. This notation documents the transition from practise without notation to the written notation of music for keyboard instruments, and it significantly supplements the material found in treatises from the milieu of the ars organisandi, which are available to us from fifteenth-century manuscripts.