Stitch holders are used to hold open stitches when not on a knitting needle.
These stitches will usually be replaced back onto a needle for further knitting.
Often shoulder stitches are put onto a stitch holder, where later on the shoulders would be seamed together. When a piece needs to be split into sections, a stitch holder can securely hold those stitches for later use.
What size you need depends on how many stitches need to be moved to a stitch holder and how big the stitches are (such as 4ply weight versus super chunky weight).
Available in three sizes