This is one more of theoretical posts. The topic of prints and optical illusions they create is very large, so I’m going to write a series of posts about optical illusions. But keep it in mind that there are some special cases which I won’t cover (I can’t cover all, because it will require a monograph). The good news is that mirrors are still exist in this world and nothing can beat the old “try and see for yourself” method.
I think knowing about optical illusions applied to clothes is useful, so there would be no unpleasant surprises if you grab something non-returnable on final sale online and then discover that it visually changes your proportions in very unflattering way.
Today post will be about stripes
There is a very common stereotype that vertical stripes are slimming and horizontal stripes are widening. But actually it depends.
Optical illusion of stripes is most prominent for high contrast stripes, for low contrast and blurred the print behaves more like solid color. Wide stripes effect is a little bit less affected by contrastness, that thin stripes effect.

There are two Cathrine Melandrino dresses from Yoox, left behaves more like dark blue (it’s slimming), for dress to the right stripes are at play (it’s making girl wearing it appear larger)
Optical illusion of stripes based on fact that our eye is lazy and it likes to travel along lines and we tend to overestimate lines along which eye travels. But there is a gotcha: sometimes our eye tends to interpret stripes as a whole and travels in the perpendicular direction.
And one more thing that should be taken into account: everything busy is perceived larger. So with very bright thin stripes with small distance between them, especially if there are a lot of clashing colors, garment will make you taller and wider no matter what is stripes direction.
If we have more of one color than another then slimming/widening effect depends a lot from prevailing color. Cool/dark color as prevailing color is more slimming/less widening then light/warm prevailing color. If the the both colors are cool/dark or light/warm and stripes are not very contrasting, then the print behaves more like a solid mix of stripes colors, and again cool/dark is slimming and and light/warm is widening.

These are two tops by Rena Langa from Yoox, you can think about left as coral and about right as greyish blue when analyzing how they would affect shape perception
This is should be considered, but, please, if you have light and warm coloring, I beg you do not opt for coolest and darkest, it can make you a little bit slimmer, but it will also make you look sick. The same about contrastness, if you have very delicate coloring (soft, light and muted), then high contrast stripes will get more attention than your face, so wear them when you want to hide.
If we have just one vertical line we usually need lighter/warmer color in the middle to get slimming effect.
Then there are wide stripes and thin stripes. Wide stripes are widening no matter what is direction (wide stripes make people overestimate distance in average by 8%). If you are X shaped then wide stripes is something to wear despite how it looks or something to use on sleeves and hems.
But not all thin stripes are equal. Look at this pictures. Most people see left one thinner than the right one.
But again it’s not universal. There is a trick that could make thin vertical stripes like this slimming: it’s gradiented distance between the stripes.
The important thing to keep in mind is print distortion on curvy body. Because when we have clinging dress we can get instead of parallel stripes something like that:

This is a dress from ASOS on a very slim and tall fashion model, but stripes distortion and their width make her body quite voluptuous, if you are already voluptuous, I would suggest to avoid such dresses like a plague
Compare it with this

Another stripy top from ASOS, but this one makes girl to look smaller despite widening stripes, just because it’s so oversized
Distorted stripes are completely different case than parallel. They create an illusion that all curves are curvier than they actually are: because we have so called 3D illusion on top of real 3D shape and often psychological effect of too small item. And too small clothes makes us larger in the eyes of viewers, even if there is only impression and actually the item feels absolutely fine*. And in this case it’s completely irrelevant what is direction of stripes. This effect is the most prominent for regular dark and light stripes, for stripes of different width and shade it’s much less noticeable.
These are photos of Darlene of Hourglassy from her post about tankinis (you can find there also some interesting thoughts and visuals about swimming bras and how they affect the look). You can see that stripes on the left got more distorted and vertical row of buttons not much of a help, because they work as an accent here drawing attention to the bust area. Top in the center has different kind of stripes and they are less distorted and work as a whole, but when you are looking along the stripes you see how underbust and waist are slimmer (stripes help us see this because they create wide dark groups), but they also make the bust to look wider by comparison, the whole look is rather flattering I would say.
So if you don’t want your bust to look bigger then avoid stripy jersey items, where stripes visibly widen or curve on your bust when an item of clothes stretches to fit. If it’s desirable effect then go for wide contrast stripes.
And don’t forget that the same effect takes place for buttocks**, thighs and tummy.
Diagonal lines are neither slimming, nor widening, but they add dynamics and concentrate attention on themselves. In some cases, if there is color blocking with colors that have different widening/slimming effect, diagonal lines can distort shape of the body, but with thin or moderately thick stripes it’s hardly the case. Although if the angle is not 90 degrees vertical stripes behave a little bit differently. Dynamics is still there, but more vertical diagonal stripes are slimming and more horizontal are widening.
If diagonal stripes meet at some point this point draws attention the most and we can get the acute angle overestimation illusion in play, so be aware.***
Combination of vertical and diagonal stripes strategically placed could be very flattering.

Todd Lynn. Photo by GoRunway via Vogue.co.uk See how point where diagonals met creates focal point of the dress
So as you can see there are a lot of factors which affects how stripes would look on you, but when you begin to think about it, it all becomes more and more clear. But there is also a shortcut: sites usually use the same models for different items, try to look for the same girl in similar (or the same) items which have solid neutral color or stripes, or different kinds of stripes, it will help you to understand what print adds to the impression.
Series continued: Prints and optical illusions. Plaid and check, Prints and optical illusions. Polka dot.
* Main culprits there are print distortion, proportions which are slightly off (sleeves which are too long for 3/4 and too short for full length, e.g. the same applies for pant legs), gaping buttons (even if it’s elastic pulling them and much larger person could fit into the garment).
**I highly recommend making photos of yourself from behind, because it can give you a completely new point of view on how you look.
*** You can look it up on Wiki, short version is: <—> make line in the middle appear shorter, and >—< make line in the middle appear longer.