How does a framework like Blueprint CSS keep your site looking consistent from one screen resolution to another?

I've been told to look into开发者_C百科 using a framework like Blueprint or 960 for my CSS.

What I don't understand is how it makes a site look consisent even if two users have different screen resolutions.

How does that work?

They specify sizes using units that are not relative to the window size. So if the user has a wide window there is a gutter and if a narrow window, a horizontal scrollbar.

Resolution is irrelevant except as a limiting factor on the user's maximum window size.