If you have an old-school bike with a 1" threaded fork (22.2mm internal diameter), this is the adapter you will need to make it compatible with a modern threadless stem so you can use a newer stem and handlebar on your vintage machine.
Simply slide the threadless stem onto the adapter and insert the adapter into the steerer (like you would normally do with the old-style quill stem). Then just tighten the stem to the adapter and tighten the quill bolt on the adapter. The adapter allows about 20mm of height adjustment.
The adapter is made from 6061 aluminum and is machined to good tolerances. Top cap and quill bolt are included. Select from 1" or 1 1/8" to match the diameter stem you are going to use. Most folks will want to select the 1 1/8" since all newer stems are 1 1/8".