Both varieties are distinguished from other football sports by their use of hard plastic helmets and shoulder pads, the forward pass, the system of downs, a number of unique rules and positions, measurement in customary units of yards (even in Canada, which mostly metricated in the 1970s, yards are still used), and a distinctive brown leather ball in the shape of a prolate spheroid with pointed ends; despite their similarities to each other, both variations of gridiron football's modern rules were developed separately from each other.