A toggle is a switch between two states: on and off. It’s found in almost every aspect of computing when there are options or preferences lists where an option can be set to either A or B. Toggles are preferred over radio buttons because they require less screen estate and come with a pre-selected default state (ON or OFF).

In software, toggles are used to implement settings, user options, and other types of configuration that can be changed and delivered immediately, without having to restart the application, re-deploy an artifact, or even re-log in to an environment. This flexibility makes toggles a key tool in most modern development and deployment processes.

The word toggle is also used in hardware: for example, the Caps Lock and Num Lock keys on keyboards are toggle switches that bind certain functions to their corresponding physical buttons. The term is also applied to other hardware switches, such as cordlock toggles that fasten a cord or drawstring.

Generally speaking, the use of toggles in an interface requires a level of user understanding and savvy to use effectively. When users see a toggle in an interface they are typically told what the current state is, and if there is a default state it should be clearly communicated as well. A common example of this is the Airplane Mode control on mobile devices – users understand that the toggle is either ON or OFF.

It is important to remember that, as a design principle, toggles should be used sparingly and in conjunction with other controls. This is because the use of toggles in interfaces may cause a cognitive overload for users. This is especially true if toggles are placed in the center of the interface and have different states depending on which side is pressed. This is why it’s best to place toggles at the edge of an interface, or alongside other controls like checkboxes and radio buttons.

In addition to this, savvy teams should be proactive about removing toggles when they are no longer needed. Some teams have a rule of adding a toggle removal task to the backlog whenever a new toggle is introduced, while others put “expiration dates” on their toggles, ensuring they are removed before they start to clutter up a production environment.

Using toggles for a large number of features can become a real headache when you have multiple environments and multiple teams working on the same codebase. To avoid this many companies opt to move toggle configuration out of static files and into a centralized store, often an existing application DB. Then they build out some form of admin UI to allow system operators, testers, and product managers to view and modify feature flag configuration.

It’s important to test all of the toggle configurations that will be deployed in a release. That includes testing the expected production toggle configuration and the fallback configuration where all toggles are flipped Off. Additionally, it’s usually a good idea to test any additional behavior that will be enabled when the toggle is flipped On.