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Draw a ruler across any webpage to check the width, height, or alignment of page elements in pixels. The MeasureIt plugin is a simple little tool that allows you to measure the size of elements in your browser.

Some instructions for Use this extension :

click the MeasureIt button in the status bar or toolbar to turn it on/off
click and drag out a ruler box
click and hold to move the ruler box
Arrow Keys to adjust size of ruler box by 1px
Arrow Keys + SHIFT to adjust size of ruler box by 5px
click and drag bottom right corner of ruler box to resize
CTRL + Arrow Keys to nudge ruler by 1px
CTRL + SHIFT + Arrow Keys to nudge ruler by 5px
click the "X" to close that ruler or click outside of it to begin drawing another ruler
ESC key to turn off MeasureIt