Today I needed a nut and washer to hold my bike tire. I had one nut already which I was able to lock on there with a 15mm socket & ratchet. So I figured this meant that the nut must be a 15mm and a 15mm washer, if that makes sense? I only had one nut and washer so I took a trip to the hardware store today and I go hunting for that nut & washer. Soon I am overwhelmed with new terminology there were metrics and hex and some other stuff I figured I needed a 15mm so the metric section must be it. I go hunting there but nothing is labeled with mm it's all +1.50 pitch or M8 I don't know what any of that means or how it relates to the 15mm diameter nut & washer. Essentially I couldn't find the right one, an employee showed me what he said was the 15mm nut & washer was but it was way too big so I knew I'd have to go back to do more research. Please explain what pitch means for mm I am very confused here. And if a 15mm socket can lock the nut onto the bike does that mean the nut would have to also be a 15mm nut & the washer?