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A battery slot in Microsoft Surface keyboard.
Preventing a mistake which cylindrical battery incorrectly put a slot on.
What is Microsoft Surface keyboard
The Microsoft Surface Keyboard is a wireless keyboard for a computer. Wireless implies the keyboard needs supply electricity by a battery. The keyboard uses two AAA batteries, which are set into the half-cylinder on the back of the keyboard(See the image). The half-cylinder is useful and Good UI for a user to set the batteries into it.
Signifier
The illustration of blue batteries on the bottom of the half-cylinder clearly shows that how many batteries need.
The metal convex( in the front of the image) and metal concave( in the back of the image) on the circle-wall implies that you connect the convex to minus side and the concave to plus one in the battery. As a result, The convex and concave prevent a rule-based mistake in which a user plans a wrong orientation of the batteries.
Affordance
The shape of the half-cylinder shows the relationship betwenn it and batteries in the sizes, shapes and number of batteries.