1:1 aspect ratio

The square has two separate ancestries. In photography it comes from medium format cameras that exposed a square negative, which freed the photographer from deciding orientation at the moment of the shot. On the web it comes from social feeds, where a uniform square made an arbitrary mix of portrait and landscape uploads into a tidy scrolling column.

Its practical advantage is symmetry under cropping. A square crops out of the centre of any landscape or portrait image without a decision about which edge to favour, and it expands into either orientation by adding padding rather than by removing content. That makes it the natural base shape for an asset library that has to serve several ratios.

It is also the only shape where orientation stops being a property. An avatar, an app icon and a favicon are all square precisely because they get placed into layouts that have no idea which way up the original was, and because a circle mask, which is what most interfaces actually apply, inscribes cleanly in a square and awkwardly in anything else.

The cost is wasted area whenever the subject is not square. A landscape scene squeezed into a square either loses its sides or floats in empty space, and a square hero image on a wide screen leaves most of the viewport unused. Square is a container shape, not a composition shape.