I'd like to know this too, I have a Brother SE-350 and the bobbins at the embroidery set up in JoAnn's didn't have bobbins the size of mine and I can't find it in my manual.

The Metro site has a short list of machines and the style they use. Some household machines use L, but others like Brother Innovis use A. There's a third style, M, but that's used for big industrial machines.

I'm quite sure I've seen a more extensive list somewhere, but I've no time to look now.

most home machines are "L" commercial machines are style "g" or "m". I have a brother and I get the poly, because there are some that come prewound that are cotton too....Hope that helped some.

One size is L, the other is A. Most use L, and the site should say which machines the prewounds fit.
If they fit a Babylock, they fit your machine.

I bought a case of black and white prewounds, and I can use them in both my Babylock nd Janome.

Also, you can go to the FAQs on the Brother site, or , if that's no help, you.

What are the models of your Brother machines?
This will help find the right pre wound bobbins for your machines.

There is a chart showing what size of Pre wounds that the different Models of each Brand of machine takes on the Allstitch website.

Not all the models of machines are listed.
Some older models can not use pre wound bobbins.

Click on the words, "Click here for the home embroidery machine bobbin size chart." that are located under the pictures that show the different Pre Wound Bobbin dimensions at the link below.

http://www.allstitch.net/content/what-size-bobbins-will-work-in-my-home-embroidery-machine-1026.cfm

EastWitch2

There are 2 sizes of Pre wound bobbins for home embroidery machines.
The size L.
And the Class15-Style A.

The Class15-StyleA is a little taller than the Size L and it holds a little more thread.

It is important to get the right size for your model of machine.
EastWitch2

OK, my machine if a Brother SE-350 and takes 15 A according to the chart shown on the allstitch site. Thank you so much to all that helped! Gonna order some next week, and some mylar and some cloud soft and some.............

If you notice at the top of the Chart is tells you what the Pre Wound Bobbin size is similar to in size to the actual bobbins that your machine uses and comes with (meaning the bobbins that you would wind yourself for your machine).

EastWitch2

I've tried prewound and really don't like them. Maybe it's just me, but I had a hard time getting them started and then they would "lock" up on me they were wound so tightly. I use my little "sidewinder". Love that thing. So fast.