These Stem Adapter Shims from Ritchey allow you to use a larger diameter stem on a smaller diameter fork steerer tube. Just a little primer here: the metric size of a standard 1 1/8" round fork steerer tube is 28.6mm. The internal diameter of a stem designed for a 1 1/4" steerer tube is 31.8mm. If you want to use your 1 1/4" compatible stem on a 1 1/8" steerer tube, you select the shim size listed as "31.8 to 28.6mm".

This type of shim was commonly seen when bike manufacturers moved from 1" fork steerer tubes to 1 1/8" steerers and most stems were then sold for 1 1/8, but you could shim down to 1", hence the 28.6mm to 25.4mm version of the this shim. Essentially these shims give you some usability across size standards if you have an older fork or a non-standard stem. Made with 6061 aluminum alloy, these shims are simple but effective with a slight weight gain of about 15 grams when you need to adapt to a lesser diameter steerer tube.