Heavy duty blocks are safe and easy to use. They offer many options for leveling single wheels, dual wheels, hydraulic jacks, tongue jacks, stabilizer jacks, 5th wheel jacks and tandem axles. Customize your levels: Stack interlocking blocks to desired height. Strong and durable blocks are available in a ten-block set and a four-block build-up set. Patented. Each set comes in a convenient storage bag.