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An overlay keyboard is a type of input device that consists of a board with a series of keys on it that are typically unmarked and not assigned to any particular meaning. It is essentially a blank keyboard that can then be programmed to have each key represent a particular input signal. This type of device is often used in the fast food industry in which each key can be assigned to a different food, to make input fast and easy. An overlay keyboard can also refer to a virtual keyboard that appears on a touch screen, allowing someone to enter input through it.
There are many different layouts and designs that can be used to make an overlay keyboard, though their functionality is essentially the same. This type of device typically consists a number of keys that can be arranged in many different ways, set atop a base. Keys are often in a grid formation, either in a square or rectangular assortment. Also, they are left blank since each one is inherently not assigned to any particular letter, number, or function. There are even some expensive keyboards that have tiny screens on each key, which can display the function of that key.
Different sheets of labels can be used with a standard overlay keyboard in order to provide a user with a guide to key functionality. These sheets are often called overlays, hence the name for this type of device. Individual stickers or labels can be used for each key, though more often one full sheet covers the entire overlay keyboard. This allows the user of such a device to simply change out the sheet when altering input configurations for the device.
A wide range of different functions and input types can be assigned to each key of an overlay keyboard, often limited only by the programming used with them. Engineers, for example, can use a keyboard like this to assign different functions commonly used in engineering software to the device. This can then make input faster and easier, allowing the engineer to use one key for a function that might require a combination of keys on a standard keyboard.
The term “overlay keyboard” can also be used to refer to a virtual input method that can appear on a touch sensitive screen. This is usually a series of keys that are arranged in a way similar to a standard keyboard that “overlays” the rest of the image on the screen. The user of this type of device can then use the touch sensitive screen and overlay keyboard to provide input.
Those overlays were one of the features that the manufacturers of the old ColecoVision video game console bragged about quite a bit. By having a controller with a numeric keypad that was designed for use with an overlay, games could be made that offered a lot of customizable functionality.
For example, the "Mouse Trap" game came with an overlay that let the user alter the maze by hitting one of three buttons to flip sections of the maze that corresponded to a color (blue, red or yellow).
The problem with overlays is that they tended to get lost. Finding a game complete with overlays for a ColecoVision or other system that used those overlays is difficult these days.