The amount of holes in buttons.

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 (that is supposed to be a button with one hole)

The cleanest option visually. Often used for thicker fabrics, the one hole button. Instead of a hole it is often a loop hidden at the back of the button.

( : )

    (that is supposed to be a button with two holes)

The two holed button is very easy to attach and is often used on shirts etc.

(::)

 (that is supposed to be a button with four holes)

The classic, the button with four holes. A tad trickier to attach, as you have to decide whether the button is stitched parallel of crosswise.

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