The 21" SINGLE SIDED Horizontal & Vertical Adaptive Rail slide on the Adjustable Bar (PURCHASED SEPARATELY) and lock in position by tightening 2 Friction Bolts on each Rail. Each 21" Horizontal or Vertical Rail offers 6 Mounting Positions to Install AIM MOLLE RECEIVERS (SOLD SEPARATELY). The AIM ACCESSORIES which allow the user to carry weapons, tools, gear, etc. are sold separately.