Frames are an HTML feature; simply speaking, it allows the designer to divide the web page into zones that can be manipulated independently of each other. Site designers are sometimes tempted to use frames to achieve a consistent user interface - the top of the page is kept constant, while the text below changes from page to page.
Frames are prone to terrible usability problems, typically when the content of the frame is too small to fit (as when the user increases the size of the text font). Designers sometimes get into a kind of recursive fix in which a framed page is opened inside of a framed page - frames within frames within frames leave no space for useful content and dismay visitors.
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