The Prelude:
The prelude is a style that is somewhat personal, flexible, and improvisatory. The rationale behind such a conception traces back to the Renaissance and Baroque periods, when very early (notated) preludes appeared and were being developed.
According to the Harvard Dictionary of Music, the noticeable qualities of the prelude are idiomatic virtuosity, rhythmic freedom, and loose thematic construction. The tiento, toccata, ricercar, fantasia, arpeggiata, tastata, and entrada are all virtually identical in style and function to the prelude.
The very early examples of the preludes are thought to be freely composed small improvisatory pieces for organ, lute, and other Renaissance string instruments, and used as an introduction to the main works or a warming up.
Unmeasured preludes:
In the 17th century, French lutenists and harpsichordists developed the unmeasured prelude. An unmeasured prelude is a prelude with neither time signature nor bar lines and often has very few note-value indications. Because of this, rhythmic interpretation is mostly left to the performer, resulting in the music being highly improvisatory.
Here is an example of Louis Couperin's unique notation for his unmeasured preludes.
Two samples of unmeasured prelude and fantasy outside France:
