Although written traces of instrumental dance music date back to the thirteenth century, the earliest extensive collections of instrumental dance music come in the form of the fifteenth century collections of bassadanza (in Italian) or basse danse (in French), meaning "low dance." That term referred to the height of the dancer's feet off of the floor. "Low dances" were generally the stuff of the upper classes and nobility.
These collections of bassdanza resemble the notation of Gregorian chant, rectangular figures on a staff, but with one difference: large strings of letters now adorned the pages.
What these manuscripts actually document were improvised accompaniments to dances over a given bass lines (or "tenor," as they were known at that time). The ensemble, know as alta capella, musicians formed something akin a guild, transferring their craft to one another via aural, rather than notated forms, hence this music's delayed entry into the literate tradition.
In the final section of the bassadanza book composer Diego Ortiz writes: "I thought I'd include the following recercadas on these bass lines that are usually called tenors in Italy, but that are mainly played as written here, in four parts, with the recercada over them." What follows is an improvisation not accompanied as it would normally be: by a cantus firmus in long note values, but a series of harmonic cells that can each be repeated in order to prolong the improvisation, if needed.
Thus, the tenor has now become what we might call a "ground bass," and forms the first harmonic-driven structure in Western art music. They create harmonic structures that are not born from voice leading but exists purely as vertical structures, and "accompany" an improvisation.