I've heard from new mitten knitters who want more info regarding peasant thumbs for gloves and mittens. I'm going to divide this into two blog posts because of all the photos.
Most charts will have either a solid line or a rectangle like above showing where the thumb will be placed. I vastly prefer patterns that have two different placements for the thumbs on each hand so I can make sure the beginning of the row is at the outside of the hand next to the little finger. That way I don't have to look at any color jogs in the pattern when I'm wearing them. Don't be afraid to move peasant thumb placements on the chart to make the mittens perfect for you.
When you get to the line on the chart that shows the thumb placement simply put the number of stitches for the thumb on waste yarn. Some patterns will tell you to knit across those stitches in waste yarn and then go back and knit the same stitches in pattern but I prefer keeping the thumb open so you can try on the mittens in progress and get the length perfect.
BTW, the chart in the photo is upside down because I'm working the Zilboorg mittens (from the out-of-print book Magnificent Mittens) the opposite direction. That is another benefit of a peasant thumb - it is worked the same in either direction.
After you put the thumb stitches on waste yarn, return to your working needle and cast on the same number of stitches you just put on waste yarn. I cast on using the backwards loop cast on and I cast on in pattern, using both colors of yarn. If you cast on with only one color of yarn you'll unfortunately have a really long float when you rejoin the row.
Once you rejoin the row you just continue knitting around. When you get to the stitches you cast on using the backwards loop cast on, you will REALLY appreciate it if you did that cast on loosely or it will be a serious pain to knit those stitches.
Now you just finish knitting to the top of the mitten. I try on the mitten and generally begin the top decreases when the mitten reaches the top of my fourth/ring finger.
Tomorrow I'll talk about picking up the stitches and actually knitting the thumb.