Horizontal design lines and details often get a bad rap for making your body appear wider.  But that ugly reputation is often undeserved.

First let's consider horizontal stripes. Bold rugby-style stripes - especially in bright colors like these - can indeed draw the viewer's eye across your body.  But if you want to make your top half look more filled out to balance wider hips, that can be a good thing, not a bad one.

And consider what happens when you add a jacket,worn open so that only a narrow band of the stripe is visible.  Now your eyes tend to "climb the ladder" of the stripes -- moving in a slenderizing vertical direction

Narrower stripes and stripes that fade from one color into another - like those in the photo below -  tend to create that ladder effect automatically (enhanced in this example by the front zipper)

Design details can also create  horizontals within an ensemble -- for better or worse.  Positive or negative placement of a horizontal detail depends on the body shape wearing it, and where that side-to-side attention is an advantage.

For a triangle body, horizontal details at or near the shoulder line balance fuller hips and thighs.  Yokes, patch pockets with flaps, broad collars all work this way.

For an hourglass, the waistline is usually a favorite place for a horizontal to call attention.   A narrow color-blended belt accomplishes this effectively. 

The point where a shirt or sweater ends (or is tucked in) forms another horizontal -- so it needs to hit at a flattering part of the body -- such as the waistline of an hourglass or triangle body, or the hipline of a rectangle or inverted triangle.  The greater the color contrast between the top and bottom garments, the more critical this horizoltal line placement becomes.

Cory's triangle figure is enhanced by moving the horizontal color break from her hips to her small waist. And the scarf brings a flattering almost-horizontal to her shoulder area.

The horizontal seam in this jacket makes the bustline look fuller.  Contrast piping or topstitch trim would make the effect even more pronounced. But if the garment was made from a print fabric, the effect of the horizontal would virtually disappear.

For inverted triangle bodies, horizontals below the waist can balance upper-body fullness and call attention to shapely legs. ( These skirts are good examples.)  For other figure types .. maybe not such a great idea.

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Consider the impact of horizontal jeans yokes and patch pockets-- especially with contrast topstitching --  on all but the most perfect backsides.  Trousers, anyone?