A sort of music that people may not be familiar with:
Prelude in A Minor, by Elizabeth Jacquet de la Guerre. It's actually originally for the harpsichord (or other keyboard), but is here played on a (replica) contemporary Spanish guitar. The instrument choice is period-appropriate, but not region-appropriate: this is a distinctively French genre (though sometimes imitated by Germans, and very occasionally by the English).
What's going on here? Well, this is an example of a 17th (and early 18th) century avant garde musical genre known as the prélude non mésuré, or 'unmeasured prelude'. They evolved out of earlier French lute music, but later were exclusively associated with the keyboard. Often (as in this case) they served as toccatas, introducing a suite of more formally-constrained music (usually dances); sometimes they were played alone, and sometimes they even took on the guise of another form (often the allemande, a dance that was often quite abstract in its music). Many are written in the ancient French tradition of a tombeau: a mournful piece composed in memory of a mentor or earlier composer, and some even reuse specific melodic motifs associated with tombeau preludes in particular.
But why are they called 'unmeasured'?
This sheet music shows why! This is a similar piece by Couperin, the composer most associated with the genre; and, as you can see... there are no bar lines! Nor are there any note values! In this piece, I don't think there's even any clear marking of which notes are meant to be played simultaneously. Instead, the composer indicates which notes are to be played, but the timing, the stress, the tempo, the manner of playing (even whether notes are to be held down) is all left to the taste of the performer.
Composers didn't completely give up the chance to give advice. In Couperin's notation - one oddity of the genre is that each composer reinventing their own notational schemes, and weren't even consistent between pieces... - he uses what look like slurs and phrase markings - although some of them are placed in impossible ways (some even become vertical lines) to guide the melody. However, the simple slur line evidently serves multiple purposes, sometimes ambiguously; often, it is really serving, it is believed, not to unite notes, but to separate them (notes bound together by slurs form groups, within which the performer is free to apply their musical taste in playing the notes, but ought not, it seems, to combine a note inside a group with one outside the goup). Other composers used other devices - often extremely counterintuitively for the unwary. Many distinguish white and black notes - but not in marking duration. Rather, white notes mark harmonic structures, while black notes mark melodic patterns. Some unmeasured preludes even appear to be measured, with bar lines... but the bar lines are really Couperinian slurs turned vertical, acting not to count the pulse but simply to group and divide notes.
This example by D'Anglebert shows a more (misleadingly!) normal-looking system. Here, he distinguishes quavers, semiquavers, and semibreves... but while the first two might indicate rhythm perhaps (though I think it may be about melody vs grace notes?), the semibreves clearly aren't meant to be played as such. (
Here, by the way, is another example by D'Anglebert, without visible score, which I include not just because it sounds nice but because
wow that's a stunning-looking harpsichord!...)
But why? What's the point of going to all this trouble - given that composers spent decades trying to devise a notation system for this genre, distinct from all other classical music, and each seemed to feel they couldn't just settle for the scheme invented by another, they clearly felt this was important. Why not use the existing mainstream western notation? Well, this music wasn't just meant to be visually unmeasured: it actively attempts to defeat the instinctive hearing of pulse and beat, to form a continuous melodic whole that exists, in some sense, outside of time. This is seen in a much wider pattern in this genre of fighting against recognisable repetition of any kind; I've seen notation of some of JdlG's works, for instance, that make clear that some sections are composed of repeating arpeggio patterns... but broken up asymmetrically by the insertion of dissonant tones, to prevent the ear from fully catching their regularity. Similarly, adjacent phrases are often intentionally of different lengths, so that even if a lazy performer attempts to impose a repeating rhythm on the notes, they can't succeed. And although it's not relating to time, you may notice that the genre is often harmonically adventurous even by Baroque standards, with many unexpected and even dissonant harmonies. And in some of these examples perhaps you've noticed the rather odd, inconclusive endings...
You can get a better sense of the oddity of this music by comparing it to 'normal' music of the time. Not only can you do this in a suite, but even in some of these preludes themselves, which juxtapose mainstream and non mésuré sections. In
this piece by Rameau, for example, the prelude itself contains, as it were, an unmeasured prelude-within-a-prelude, leading into a much more typical Baroque second half (at 1:35). Finally,
in this piece Couperin begins in the unmeasured style, before returning to normality (2:50)... only to fade back into unmeasuredness to end (4:20).
Anyway, I didn't really have anything to say about it. It just struck me that this entire episode is a really unusual moment in the history of Western music, which many people are probably completely unaware of, so I thought it was worth mentioning...