The most commonly used and traditional format for artwork is the rectangle. Its horizontal nature relates to the earth's horizon, creating a grounded and calming effect and is by far the most popular for landscapes. When turned vertically, the rectangle relates to the human head and is often referred to as the portrait format.
The symmetry of a square creates a compositional challenge and I encourage you to give it a try. The basic rules of placement don't really apply to the square. If you use the rule of thirds for the center of interest it looks too even so this frees you up to throw the rules out the window and have fun.
Of course, when working with watercolor paper we are free to make our painting any size we want but canvas painters have always been limited to the sizes available to them. In the last few years there has been an increase in the popularity of square canvases and they can now be found in almost any size you want.
With my collage and mixed media art I seem to gravitate towards the square canvases - maybe because they always seem to have them on sale at Michaels:)
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