Define what a triskelion is .
A triskelion shape was used in the design of RCA 's " Spider " 45 rpm adapter , a popular plastic adapter for vinyl records , which allows larger center - holed 45 rpm records ( commonly used on 7 " singles and EPs ) to spin on players designed for smaller center - holed 33-1/3 rpm records ( the standard for 10 " and 12 " LPs ) .