This is a double chainwheel which enables small change in ratio to be effected by stopping the bike and swapping your chain from one to the other. There is a two tooth difference which is small enough to be accomodated by most horizontal drop outs.
Additionally, for the full blown double/double system (which mates a double freewheel with a double chainwheel), there is a variation in this product with a three tooth difference - practical in this instance because by running a large sprocket with small chainwheel and vice versa you cancel out the difference in chain length between ratios.
As you would expect from White, there is the best in breed materials (case hardened steel with nickel plating), machined working parts and sealed bearing. There are 36 points of engagement. The bearing is replaceable so your hub is protected from an early grave, and as with all White stuff it is made in the United States. Fits traditional English hub thread.
White usually comes quick even when marked "out of stock".