Blum Tandem Premium Undermount Slides Basic Self Closing 3/4 Extension For 21" Drawers 100# Class Full extension for 3/4" drawer side. 21" Drawer requires 21-3/4" minimum interior depth. For Inset applications increase Minimum Interior Depth by drawer front thickness Undermount to drawer - Side-mount to cabinet Rear-mount with optional bracket, Search: Blum Rear Mount Bracket Locking devices are required, Search: Blum Locking Device For Tandem Drawer Slides