Important aspects of my discussion of Busnoys’ “In mijnen sijn” are most adequately represented by the musical editions appended to this article. They include separate editions of the song’s only two complete sources, which date from the first decade of the 16th century. For anybody who wants to perform the song, these sources raise some thorny questions about how to understand the music. The editions include my attempt to answer these questions by means of a restoration of “In mijnen sijn”. The process of restoration highlights some issues of importance to our perception of the development of compositional practice in the second part of the fifteenth century. These issues concern the extent and meaning of the roles of key signatures, strict canon techniques and the development of polyphonic settings of popular songs. Furthermore, in my opinion this Flemish song has not received the attention it deserves from musicology.