M-LOK, for Modular Lock, is a free licensed firearm rail interface system developed and patented by Magpul Industries. M-LOK allows for direct accessory attachment onto the "negative space" (hollow slot) mounting points, and is a competing standard to VLTOR's open sourced KeyMod system for replacing the ubiquitous Picatinny rail in some applications. Compared to the Picatinny rail system, both M-LOK and KeyMod enable the user to have a slimmer, lighter, smoother and more fenestrated handguard/fore-end with accessories mounted only where needed, whereas a Picatinny handguard typically will have rail slots for its whole length, resulting in a heavier and bulkier handguard with sharp edges and poorer barrel ventilation.