The Melvin bolts to the frame where the derailleur would normally go and allows the use of a one speed rear wheel on bikes with vertical dropouts.
The Melvin also works with multiple front chain rings while maintaining proper chain tension. Of course this is no longer a single speed, but can be very handy for loaded commuting. Because the Melvine uses two pulleys, two chain rings with a maximum difference of twenty teeth can be used up front