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The EM is a measurement just like PX or PT. One EM is equal to the pixel measurement of the current font size. So, for example, 1em on a web page with a font size of 16px equals 16px. The direct relationship between font size and EM allows us to build layouts that scale along with the font size. This means that a visitor with a browser font size set to "large" will see a perfectly scaled version the layout rather than a skinny layout with really big letters.