Compatible cartridge for Brother LC51BK (black). Unique cartridge by BISONTEC holds TWO TIMES the ink volume of the original Brother LC51 cartridge and therefore has TWO TIMES the page yield! Its like getting 2 cartridges in 1. Works exactly the same as the original fits exactly the same size and shape. How can we fit 2 x the ink into 1 cartridge? It's Simple! The original cartridges have a lot of wasted space on the cartridge itself we just utilize all the space and fill it with more ink. You get 2 x the ink at a lot less price than buying an original or even a lot less price compared to buying other compatible cartridges.