Gaping pockets are a common affliction on plain-front trousers, and are almost always caused by tightness in the hips- the tightness causes the pocket to pull open and form what some say look like dog's ears on the side of your hips. Never an attractive feature.
When selecting a coat size, we advise to fit the shoulders first, then worry about the rest; when fitting trousers, it is best to find a size that fits correctly in the seat and adjust the waist to suit. This is one of the reasons. There is a possible fix, however.
Tightness in the front of the thigh is often a cause of this and can be remedied by dismantling the waistband and working about 3/8" of fullness across the front of the trouser, taken from the outlet at center back, shifting everything forward. An extra dart above the back pocket (there is usually only one) is helpful for prominent seats (but difficult to add to a finished garment), and a dart in the front helps with very muscular thighs.
We don't often see this front dart on ready-to-wear, and when we do, it is usually in the wrong place. The most aesthetically pleasing spot for this dart would be right under the belt loop where the crease would intersect the waistband were we to extend the crease all the way up the front, as illustrated.
Anatomically, however, this placement is incorrect. The hip is somewhat hollow at that point, and the fullest part is toward the side. By placing the dart next to the offending pocket we can add some extra room for the thigh and correct the problem.
The fullness and extra dart can be seen on this outstanding trouser, part of a suit made by Leonard Logsdail for an evidently athletic client who donated it to this blog. You can see the proper dart placement (some prefer to approximate the angle of the pocket for aesthetic reasons) as well as the slight puckering along the waistband where the fullness has been worked in. Bespoke tailors will be aware of this but some alterations tailors may not so if you are a former (or current) hockey player who has trouble with trouser fit, you may want to bring this up with your alterations person.
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