The other day I had a "learned something new today" moment because I wasn't aware a seam gauge can be used like a compass to mark circles and scallops. But, yes, it most certainly can!
All you need to do is anchor it with a pin through the hole in the slider, insert the tip of a fabric pen in the hole at the end of the gauge, and then rotate the gauge to trace the circle or scallop.
The two things to pay attention to are:
- If you need an exact diameter for a circle or specific height for a scallop, the hole at the end isn't actually at the very end and you'll need to accommodate for the one or two millimeter difference.
- Make sure nothing is in the way of the gauge rotating around! It's easy to knock stuff around or onto the floor and end up with a wobbly looking circle if there is something in the path of the gauge while it's rotating.
A seam gauge is one of those sewing tools I think of as a sewing necessity and that was before knowing it can also be used to make circles! Mine gets used almost daily for something (often a Grocery Bag or Basket Bag) and I have several of them since they are easy to bury under fabric or misplace in other ways.