The Borough line of bags from Brompton are all intended to be mounted on the Front Carrier Block of a Brompton bicycle. They differ from one another depending on size, color, and whether their design is waterproof-specific. 

The Brompton Borough M (Medium) Bag is a medium-sized bag perfect for carrying the daily essentials you bring on your commute. It's dimensions are 15" wide by 10" tall by 5.5" deep, amounting to a capacity of roughly 14 Liters. To fit loads slightly over 14L, simply leave the roll-top closure of the bag rolled down only part way. The bag's outer shell is made of 915D Polyester Twill and it is lined with 210D Nylon. YKK zippers and Duraflex hardware and a fully waterproof Hi-Viz rain cover complete the ensemble of durable features. The Borough M Bag also features a rear-facing (i.e. pointing toward the rider while mounted on a Brompton) zippered pocket. The interior compartment has a laptop sleeve and a mesh zippered pocket.