Check box

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Sample check boxes with two selected

In computing, a check box is a graphical user interface element (widget) that permits the user to make multiple selections from a number of options. Normally, check boxes are shown on the screen as a square box that can contain white space (for false) or a tick mark or X (for true), as pictured. Adjacent to the check box is normally shown a caption describing the meaning of the check box. Inverting the state of a check box is done by clicking the mouse on the box, or the caption, or by using a keyboard shortcut.

[edit] Tri-state check box

Some applications call for a check box that adds an additional state to the two states (selected and not selected) provided by a normal check box that can be shown by greying-out the check box. This can be useful in tree views where the two primary states indicate a state the user chose while the third state indicates an inherited value.

For example, if a tree view is presented to select all files on a computer to compress (or send via FTP or any other numerous actions) then if a user desired to send all files then it would be very tedious to select each and every file for inclusion. Checking a box for a directory would check all directories and files underneath it. Now, if one of those sub-directories or sub-files was unchecked the directory has both checked and unchecked items, thus your third state – a state that is in between checked and unchecked.

The third state can be represented by shading out the checked check box. A click on a shaded out check box could have two reactions:

all sub-directories and sub-files would all be rechecked again
all sub-directories and sub-files would be unchecked

Also, clicking on a file in a directory would mark the directory as checked if there is the one and only file in it or the directory would be in the third state for having some checked and some unchecked items.