Brooks Double Rail Seat Adapter

Originally conceived in the late ‘70s to allow the use of double-rail leather saddles on the very first MTBs, the Brooks Sandwich Seat Adapter remains a simple way to fit your favourite classic saddle to modern seatpost designs. Compatible with all Brooks double-rail saddles, including the B18, B66, B66S, B72, B135, B190 and most micro-adjustable seatposts on the market.

The lil video from Brooks below clears things up nicely if you're not sure what this all means.