What exactly is Muslin fabric?
 

I'm wondering how premium muslin is different from regular cotton.

Supposedly, it's the weave. There is a permaprest muslin that has some poly in it. I use it for everything, even clothes.
https://en.wikipedia.org/wiki/Muslin

Most of the premium muslins have a higher threadcount (200). It is very nice to work with, doesn't wrinkle as much and is a great option to stretch out your stash. :)

Be careful some have poly in it, read the bolt end and feel the muslin to determine if you like it. The one's that are good are quite nice, but not the others.

Tartan ... do you buy your muslin here in Canada? Where?

I couldn't understand it when I read here on the QB that people were using it in their quilts, as any that I have ever seen here is the old-fashioned-almost-impossible-to-iron-out-the-wrinkles" type. A friend in the USA sent me some and I soon understood why they do use it in their quilts. Only thing I did not like in working with it was that it is quite fine, so if working with darks in the same project, they can show thru.

I'm using a moda muslin I bought and love it. I'm using it as a background. It's a creamy color and goes nicely. It was like $6 a yard. Perfect

I don't go cross border, so like you often use WOWs or WOCs.
Have you done your Len's trip yet? ... they usually have lots, including widebacks!

I bought a bolt of "Roc-Lon" permanent press muslin that is preshrunk and 100% cotton.......can't remember where I got it but it might have been at JoAnns with a coupon or when I use to have a license from my wholesaler.

over 100+ years ago, muslin was the top of the line of fabric, or so I've heard...tightly woven and used in wedding dresses, etc.