The Denim Culprit

The gape usually results from picking the wrong jeans, especially when you have fuller hips and a slim waist. The gape is the undesirable gap or space between your lower back and the back waist of your jeans which is especially noticeable when sitting, bending over or squatting.
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Style Fix

Find low-rise jeans that start at the hips instead of the waist so you don’t have to worry about fitting both waist and hips. You can also find denim with a contoured waistband since it will follow your curves and prevent gaping at the waist. Another good idea is to find pants that sit a little higher in back but still low rise in the front. This is how we make our jeans at indi. Not only will they hold in your tummy but they are cut to fit your waist as opposed to your hips.
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See Also

Muffin Top
Rise
Exposed Butt Crack
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References

Fashion Dilemma’s Q&As; http://nz.lifestyle.yahoo.com/b/lifestylechannels/1968/fashion-dilemmas-qas
Why your pants don’t fit, http://shine.yahoo.com/channel/beauty/why-your-pants-dont-fit-494548/
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