These split ring markers are a simple open loop with pointed leg that will stay in place while you're working - easy to remove and reposition while crocheting or knitting - creating increases, counting rows or repeats, or working cables. The steel construction is sturdy but flexible so the opening can be adjusted to accommodate the size of yarn or collection of stitches being marked. They are nylon coated steel, so they cling to any magnet.
CocoKnits has a blog post about "4 Different Ways to Use Split Ring Markers" that you might find interesting.