Zippers and velcro fastenings have replaced laces and buckles at the calves as well. Other types of fastenings include zippers, Velcro, and magnets. The thrifty still replace zippers and buttons and sew up ripped hems. The US Navy used a slightly different model that featured slanted pockets with zippers. Hooks and eyes, buttons, zippers, and snaps were all used to fasten clothing. Some jackets have double-separating zippers with two sliders on the tape. Comforter covers are similar in principle to pillowcases, usually closed with zippers or buttons. Dressing skills are variable and usually limited to items of clothing without buttons or zippers. Some of the road crew used their jacket zippers to create sound. Other styles, particularly shotguns, usually use full-length heavy-duty metal zippers. Zip-Pak developed Polypropylene compatible zippers in 2003. Its "skin" measures , and is held in place by 1,000 snaps and 250 zippers. Zippak produces plastic zippers for the food packaging market. Cole swimsuits used latex, which meant zippers were no longer necessary. Petroleum jelly can be used to lubricate zippers and slide rules.