Spacer Blocks are used to level a Roof Rack system on a car roof that isnt level. Spacers are placed between the leg unit and the cross bar to raise the cross bar height. Each Spacer is 10mm in height. The LHS2PAIR includes 4 10mm Spacer Blocks and 2 M10x45mm bolts. The LHS2PAIR is designed to raise 1 Heavy Duty bar 20mm. The LHS2PAIR Spacer Blocks are only compatible with Rhino-Rack Heavy Duty cross bars.