These are defined places where the layout of the page shifts from one configuration to another. This is determined by the content within the page and not necessarily set to something based upon a set device’s width. For example, 480px might be a good place to set your first breakpoint for tablet, but if the content really suggests something much later (based on how it looks when at that point) you should set it at that point, whatever it is. For example, 527px would be a perfectly fine breakpoint if that is what your content dictates.
Quote:
You can Google syntax, you can’t Google understanding. — Trish Ladd