[00:02] chord naturally lives on a mobia strip here we're not going to draw any naturally think of all of the musical notes as living on a circle like this so every two note chord effectively looks like an unordered pair of points on this
[00:16] circle and the question is what mathematical space describes that you might start by labeling all of the points on a loop with values ranging from 0 to 1 because it's a loop it ends where it starts so the labels 0 and one
[00:29] would have to really refer to the same point then a pair of points would have a pair of numerical labels that you could think of as XY coordinates describing some single point in this unit Square here of the XY plane but again because
[00:43] 0o and one really refer to the same point what you should do is glue this left edge of the square to the right Edge since really they refer to the same thing likewise you'd want to glue the bottom Edge to the top Edge since
[00:56] y-coordinates of 0 and 1 are really the same thing and when you glue all of those together what you end up with is the surface of a donut known as a Taurus but this isn't really the answer that I want what I asked for was an object that
[01:08] encodes unordered pairs of points that is if you swap the two points like swapping two musical notes it should really be considered the same thing in our unit square if you glue every point with coordinates X Y to the point YX
[01:22] what it looks like is folding along this diagonal after that you still have to glue the arrows together as a way to remember that zero and one refer to the same thing but now it feels impossible the trick is to cut along another
[01:35] diagonal adding new arrows when you do to remember to glue it back together this lets you glue those purple arrows and now to stitch together the remaining ones you need to introduce a half twist so the true answer here is a mobia strip
[01:49] this was a construction that came up in a recent video where we use this fact that mobia strips naturally encode unordered pairs of points in a genuine mathematical proof and it's related to a classic solved problem