Why do some framed pieces have a mat cut with a wider bottom?

This is an older technique based on the “rule of thirds.” Generally speaking, most artworks have the focal point located in the lower third of the image. So, if the matting is cut to the same width on all four sides, the bottom-heavy “weight” of the image tricks your eye, making it seem as if the image is sinking down into the frame. This was compensated by “weighting” the bottom of the mat in order to make the image seem more buoyant within the frame.

Today, there are very few rules in framing design and the trend more often tends to utilize a mat width that is consistent on all four sides. Still, you can frequently find many traditional photographers using the bottom-weighted method. There is no right or wrong answer—it ultimately depends on your own style and what suits your artwork.