Icon in computer terminology is a pictogram which serves the user to navigate through various files and software tools in a computer. So computer icons were born along with the computers as nothing can be accomplished on a computer without an icon. Windows and Apple continue to use these formats still while UNIX and Linux stopped using specialised icon formats and started to support PNG format. An icon was generally meant to be in square shape. A 16x16 icon would not be able to serve the purpose.
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