The recent discovery of medieval murals of more than twenty angels playing musical instruments, on the vault of St. Catherine’s crypt in St. Stephen’s Church in Koufiim, Central Bohemia, represents a rich contribution to organology. The Koufiim murals are considered the largest fully preserved set of images of music instruments from the High Middle Ages in the Bohemian Lands. The paintings were created at the beginning of the 15th century. Around the mid-15th century they were whitewashed, and thus spared later changes; it is therefore possible to believe them to be faithful documents of their time. The murals represent commonly used, as well as rare, instruments. Of greatest importance here is the mural of the tromba marina, enriching current knowledge about the use of this instrument from geographical and chronological points of view. With the help of a detailed description and comparison with other period sources, the article attempts to shed more light on music instruments used at the turn of the 14th and 15th centuries.