If you measure the distance between teeth on the cassette or chainwheel, it will be 1/2 inch - it is the standard so chains, freewheels, cassettes and chainsets will work together (everything else being equal). But why 1/2” I hear you ask. Well, it was set by the British bicycle industry many years ago and everyone else followed suit and still do to this day. Shimano, as usual, had other ideas. Why not make it 10mm instead and thus all the parts of the drive train are smaller and lighter for the same gear ratio. So they made all the parts and they were for sale for a while, and obviously not compatible with anything else on the market. It lasted a few years, I think. Bits come up for sale on eBay now and again.