Abstract This article examines the préludes non mesurés composed for the guitar in the second half of the 17th century. From François Martin’s collection of suites in 1663 to Robert de Visée’s second book of guitar music in 1686, guitarists published nearly 50 preludes, several of which are rhythmically free or entirely unmeasured. Indebted to the lute and harpsichord repertories, the guitar prelude was nevertheless shaped by the notational and performance practices of the instrument. A review of the printed sources reveals four types of guitar prelude current in the second half of the century: the fully measured prelude; the precisely notated but rhythmically free prelude; the unmeasured prelude; and the strummed, unmeasured prelude with variable rhythm. This last type, which instructs the performer to strum each chord an indeterminate number of times, has no exact analogue in the other instrumental repertories of the era. Comparisons are also made to unmeasured or rhythmically free preludes in manuscript sources including the ‘Gallot’ manuscript. Together, these pieces attest to a brief but avid interest in unmeasured music on the Baroque guitar.