Radio buttons are arranged in groups of two or more and displayed on screen as, for example, a list of circular holes that can contain white space (for unselected) or a dot (for selected). Each radio button is normally accompanied by a label describing the choice that the radio button represents. When the user selects a radio button, any previously selected radio button in the same group becomes deselected. Selecting a radio button is done by clicking the mouse on the button, or the caption, or by using a keyboard shortcut.
It is possible that initially none of the radio buttons in a group is selected. This state cannot be restored by interacting with the radio button widget (but it may be possible through other user interface elements). When used in an HTML form, if no button in a group is checked, then no name-value pair is passed when the form is submitted. For example, for a radio button group named Sex with the options Male and Female, the variable Sex would not be passed, even with a blank value.
Radio buttons were named after the physical buttons used on older car radios to select preset stations – when one of the buttons was pressed, other buttons would pop out, leaving the pressed button the only button in the "pushed in" position.
Version 6 of the Unicode standard includes a character designated to represent a radio button (🔘) at code point 128,280 (U+1F518), found in the Miscellaneous Symbols and Pictographs section. Similar characters are the circled dot operator (U+2299), fisheye (U+25C9), and bullseye (U+25CE).
See also 
- W3 HTML 4.01 Specification
- Sun Java Programming Tutorial
- A usability column on when to use radio buttons and when to use check boxes
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