What is a chainless foundation and when is it worth the trouble?

Chain 2. Insert the hook into the first chain, yarn over, pull up a loop, yarn over and pull through one loop only — that single loop is the 'chain' part. Then yarn over and pull through the remaining two loops, which completes a sc. You now have two stitches: a real sc and its built-in chain base.

For the next stitch, insert the hook under the two loops at the base of the stitch you just made rather than into a chain. Every subsequent stitch is built the same way: the base loops of the previous stitch are your insertion point.

The foundation row takes roughly 1.5 times as long as a chain plus a plain row, because you are doing two things per stitch. What you save is the entire second row, plus the tightness problem: a chainless foundation stretches 30-40% more than a chain edge on the same yarn.

It also gives you a clean bottom edge with no ridge. A chain-based start leaves a visible line of chain bumps along the bottom of the work, which shows in any garment where the hem is exposed.

Amigurumi, round pieces and any base that will be joined to something else: the extra stretch works against you because the piece needs to hold a fixed shape, and the base is hidden anyway.

It is also awkward in very thick yarn, where the base loops are hard to find, and in very fine thread, where the two-part stitch becomes fiddly at 1.25 mm. Between about DK and worsted it is at its best.