Toggle () is an English word that means “switch or alternate”—for example, if you’re video chatting with two friends at once, you might toggle back and forth between their screens to see them both. But the word is also used as a verb: “to toggle” means to move back and forth between two states, as in, “She’s toggling between her French homework and her Spanish homework.”

The earliest use of the term was for a pin passed transversely through an eye or loop in a rope so as to bind it temporarily to another chain or rope; now it refers to any kind of fastener, especially one that can be easily engaged or disengaged. It’s often used in software design for a button that can switch between different settings or options. For example, a keyboard may have a toggle that turns its Caps Lock or Num Lock function on or off.

In software design toggles are the preferred way to manage system settings or preferences, because they’re generally more clear than radio buttons and require less screen estate. But it’s important to remember that a toggle is still a control, and it should be treated with the same level of caution as any other control. It’s important to have clear labels that explain what the toggle does, and which state it is in right now. It’s also a good idea to make the toggle label large enough so that it’s easy to see, even in a small form factor.

A toggle’s configuration can be static (like in the case of a Canary or Champagne Brunch release) or dynamic (like in the case of an Experiment Toggle). For the former, it’s generally safe to test the toggle configuration as it will be deployed in production, and the system should always be able to fall back to the default state. It’s also a good idea for teams to occasionally run some tests with all the toggles in the system flipped On, and other tests with all of the toggles flipped Off.

If you’re using a more advanced feature flag system that supports runtime re-configuration then it can be useful to expose the toggle’s configuration through an API endpoint. This can be helpful for reducing the cycle time in your validation testing process and for enabling the ability to perform more complex manual or exploratory testing.