Replacement cogs for single speed coaster brakes, used on all Dutch bicycles and quite a number of non-Dutch bikes as well. Available in 16T, 18T, 19T, 20T, and 22T sizes.
An easy way to "hot rod" your Dutch bike - to increase your speed to output ratio, choose a lower tooth count than your existing cog, and to enjoy a more relaxed ride choose a higher tooth count.