So, we're going to talk about compatibility issues with bicycles.
Bicycles basically, one thing that's universal, almost universal, is that most of everything is metric on a bicycle. So, the different bolts, screws, nuts all that, is metric. The thing that's not universal is, bicycles have many, many different standards. There's the Italian standard, the Swiss standard, an English standard, a Japanese standard. So, when you're buying bicycle parts, make sure that you either talk to a professional, or you do your research and make sure that you get the right parts, because there are definitely a lot of different issues.
Especially for example, bottom brackets. There's an English thread, so it threads on a certain way. There's an Italian thread, which threads on a different way. So, if you try to interchange those two, you can either strip out your bottom bracket, strip out your frame, or you can just spend a lot of money on something that's not going to work.
The best thing to do, is to walk into your local bike shop and start talking to them about different compatibilities for your individual bicycle.