My girlfriend asked me today why her 15" computer monitor looks about the same size as her 13" tv.

My understanding was that with tv's, the size of the tv is just the measurement in inches from one corner of the viewable area to the opposite corner. I think this also is true of lcd monitors. A 17" lcd monitor actually has a diagonal length of 17" as I understand it. So why is it that crt monitors dont follow this rule. Do they use a different measuring system for them and if so, what is it? Why do crt monitors measure about 2 inches shorter than they are actually quoted as? For example, why is my girlfriends 15" monitor the same size as her 13" tv?