Toggle button show “Current state” vs “Will be changed state”

Question about buttons that toggle between two states. (Think Play/Pause, or Shuffle/Regular Play.) As the title says, should the toggle show it’s current state or the state to which it will transition?

Cooper and Reimann (2003, pp. 341–2) treat this subject under the heading “Flip-flop buttons: A selection idiom to avoid”. I strongly suggest to consult this book as I will only present an excerpt:

Flip-flop button controls are very efficient. They save space by controlling two mutually exclusive options with a single control. The problem with flip-flop controls is that they fail to fulfill the second duty of every control — to inform the user of their current state. If the button says ON when the state is off, it is unclear what the setting is. If it is OFF when the state is off, however, where is the ON button? Don’t use them. Not on buttons and no on menus!

The authors (and I think of these as an authority on the subject) presents two possible solutions: You should either spell the button’s action out as a verb phrase (e.g., Switch to portrait mode, thereby sacrificing some of the saved space) or use some other technique entirely (e.g., two radio buttons).

Reference: http://ux.stackexchange.com/questions/1318/should-a-toggle-button-show-its-current-state-or-the-state-to-which-it-will-chan

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