ornamentation in keyboard music of William Byrd

Question

I'm working through some tunes by Byrd, and am wondering about the ornamentation, the two diagonal slashes through the note stem.

As far as I can tell, we don't actually know what the composers of that period meant by them, although there are some long and laborious extrapolations out there. They may be upper or lower mordents, or trills, and left out entirely or added according to the taste of the performer.

Is that about right? Is there any guidance on how to approach them besides just doing what you think sounds good?

Answer 1

From "The ornamentation in the Fitzwilliam Virginal Book with an introductory study of contemporary practice":

The single- and double-stroke ornament signs appear at first glance to be indiscriminately scattered over the music without purpose. Research into their use reveals them to be employed systematically, besides being decorative elements which add brilliance to the music. The frequency with which they coincide with the pulse unit and the rhythmic pulsation created by it, together with the profusion of their occurrence, make these signs a unique phenomenon in late sixteenth-century ornamentation. Their interpretation remains a difficult issue to clarify. The evidence assembled in this study points to a classification of the strokes according to the accenti e trilli principle. The single stroke can then be interpreted as a slide (from a third below the main note), and the double stroke as a tremolo or tremoletto - the most common sixteenth-century ornament. Its mirror-image, the mordent, is occasionally more appropriate in certain contexts, and in cadences the double stroke followed by a two-note suffix most likely signifies a groppo. (emphasis mine)

The Fitzwilliam Virginal Book contains a number of pieces by William Byrd, and the study quoted above understands the manuscript "as representative of late sixteenth-century practice."