The JBL VT4888-AF Array Suspension Frame is crafted of 6061 heat-treated aluminum. It includes 11 attachment holes for shackles, each fitted with bronze bushings for long life. These holes are set on approximately 4" centers. Each hole has an ID (inner diameter) of 25.4 mm (1"). Array frame is fitted with SAE grade 8 bolts, 7075 grade aluminum receiver blocks and steel quick-release pins with stainless steel restraining lanyards. The VT4888-AF can also be used to ground stack up to 6 enclosures and also used with VT4882 subwoofers.