Trusted Grosgrain Ribbon Source

[Dixiecat]

Here's a very good source for the difference between petersham and grosgrain:
http://www.afashionablestitch.com/20...-vs-grosgrain/

Petersham is an absolute must for those curved edges like bonnet bands. Where grosgrain will pucker, petersham will shape due to the unlocked edges. It'll make for softer looking cockades as well.

In my opinion, if you're spending the 20hours to knit a bonnet with good yarn, then go all the way and use the petersham. Definitely one of those touches that make the difference.