An internal frame mounted on adjustable elastomer shock absorbers on all eight corners. These revolutionary shocks are designed to control the effects of shock and vibration on three axes. The "sway" space around the internal frame provides natural air conditioning -- very important for delicate equipment. Inner rack chassis depth is 20" rail to rail with pre-threaded rack rails for standard 10-32 hardware that is included.They are half the weight of the old wooden cases.