Internal geared hub - only one cog on the outside. Butcher it so it doesn't freewheel - hurrah! Instant multi speed fixed! The conversion process always looks concerning to me though. Sturmey Archer actually made a 3 speed fixed hub, the ASC. You can buy one, if you want. Or, for the same sort of money, you could have a custom frame made to measure by Vanilla. But, you know. Whichever one you think is the better use of money!