Originally Posted by pack

freewheel; is attached to a hub that has an outside thread on the side of the hub. There are two diffrent thread sizes (to my knowledge) a flip flop hub has both sizes on either side, so on one side you can fit freewheels 16 tooth and upwards and the other side you can fit sizes from 13 (odyssey) to 16 tooth. in various thickness.
some hubs have only the larger thread and can only fit freewheels 16 teeth and up.


casset; is a hub with a separate driver containing the rear cog which is threaded externaly from the hub. because the driver is separate and not threaded on the outside casing of a normal hub, you can have less teeth on there and still have bearings there.
you can buy one piece drivers down to 9 teeth to fit these hubs

im not sure if this makes sense, but it does to me