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.
A number of icon builder utilities exist allowing you to create icon images, but the vast majority are online utilities with little functionality or customization. In Photoshop, you would first need to design the icon from scratch, whereas Iconion icon converter gives you a library of icons to work with.
Today, a simple square icon with as little complexity as possible tends to work best in most situations by helping the icon stand out against content whether text or visual. On the left is a palette of font glyphs ('Select Icon'), divided by font name. This area shows you the icon you've designed, along with all of the glyphs you've selected for use in this icon. Each icon can show one glyph, but you can create one style and save dozens of glyphs in that style at the same time.
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