Question about proper naming conventions

I've been working on a proper naming convention for my project, and I've been reading a lot about the different conventions used in different languages. However, there's one thing I don't quite understand. In some languages, the convention is to use camel case, proper case, or underscores to separate words, but some words are never capitalized even if they are nouns. For example: "window" or "event". I'm asking this over proper naming conventions because I don't want to name my variables with a capital letter unless they should be capitalized.