It’s a common belief that if you want to appear slimmer than you actually are, you should wear clothes with vertical stripes. The classical pinstriped business suit would be an example of this sort of clothing masquerading a few extra pounds.
It turns out that this folk belief is fundamentally wrong. A square composed of horizontal lines appears taller and narrower than an identical square made up of vertical lines.
This illusion was discovered by Hermann von Helmholtz in 1925, and is also known as the Helmholtz illusion. Helmholtz’s explanation of the illusion was that a filled out area looks longer than an unfilled area of the same size. His thought was that the figure with horizontal stripes looks filled and hence longer from bottom up, whereas the square with vertical lines looks filled and hence longer from left to right. This then generates the illusion that the square with horizontal lines is taller and slimmer than the same-sized square with vertical stripes, which looks short and fat.
Although Helmholtz mentioned the application to fashion, he didn’t test whether the illusion would persist when actually applied to human figures. Over the years researchers, fashion designers and many others have speculated whether the illusion was an artifact of the two-dimensional square representations that had been used in Helmholtz’ original studies.
In 2009, however, British psychologists Peter Thompson and Kyriaki Mikellidou followed up with similar studies using three-dimensional female models. And lo and behold! The illusion persisted.
When two people are the same size, a person wearing a horizontal-striped dress appears to be the thinner of the two. In order for the them to appear to be the same size, the person wearing the horizontal stripes would have to be six percent wider than the person dressed in vertically-striped clothing.
So, you can throw out the pinstriped suit and get the clothing with horizontal stripes.