What is the difference between single width and double width fabric?

The difference between single width and double width is simply the width. The single width is just over half the width of the double width. I.e. single width is 32" opposed to double width which is 54" wide.

Also, single-width is woven with a traditional shuttle selvedge and the double is woven with a modern tuck-in selvedge.

The reason we offer two different widths is due to some tartans being non-matching (uncentred) tartans this is when a tartan is available in a single width.

It may be worth noting that many kilt makers actually prefer single-width as they don't need to split and join the fabric.