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In computing, a checkbox (check box, tickbox, or tick box) is a graphical user interface element (widget) that permits the user to make multiple selections from a number of options (contrary to a radio button where only one choice is possible) or to have the user answer yes (checked) or no (not checked) on a simple yes/no question. A marked/checked checkbox: ☑. A unmarked/unchecked checkbox: ☐.
Normally, checkboxes 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. A caption describing the meaning of the checkbox is normally shown adjacent to the checkbox. Inverting the state of a checkbox is done by clicking the mouse on the box, or the caption, or by using a keyboard shortcut, such as the space bar.
Tri-state checkbox 
Some applications use checkboxes that allow an indeterminate state in addition to the two provided by a normal checkbox. This third state is shown as a square or dash in the checkbox, and indicates that its state is neither checked nor unchecked. This is most often used when the checkbox is tied to a collection of items in mixed states. The indeterminate state cannot usually be selected by the user, and switches to a checked state when activated.
For example, a checkbox presented to select files to send via FTP might use a tree view so that files can be selected one at a time, or by folder. If only some of the files in a folder are selected, then the checkbox for that folder would be indeterminate. Clicking on this indeterminate checkbox would select all or, less commonly, none of the contained files. Continuing to click on the checkbox would alternate between checked (all sub-directories and files selected) and unchecked (no sub-directories or files selected).
Some tri-state checkbox implementations allow the user to toggle among all states, including the indeterminate state, by remembering the mixed state of the items in the collection. This serves as an undo feature.
Not actionable 
Setting or clearing a checkbox changes the checkbox's state with no other side-effects. Violating this guideline by associating additional actions with the change of state frequently confuses users, because they are used to configuring data in entry controls such as text boxes, radio buttons, and checkboxes and then invoking an action control such as a push button to initiate the action to process the data.
One common exception is to enable or display additional user input elements that are only relevant if the checkbox is either checked or unchecked.
In Unicode, empty and filled checkboxes are represented by code points U+2610 (☐, 'BALLOT BOX'), U+2611 (☑, 'BALLOT BOX WITH CHECK'), and U+2612 (☒, 'BALLOT BOX WITH X').
See also 
- "Check Boxes", "msdn", Retrieved 2010-02-05
- Nielsen, Jakob, "Checkboxes vs. Radio Buttons", "Jakob Nielsen's Alertbox",Sept. 2004. Retrieved 2010-02-05
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