Thirsties covers are a great example of a cover that has leg gussets. Link. If you look on the inside of where the leg opening is, you notice an extra, smaller bit of fabric & elastic that actually sits against the baby's leg. It makes for a better fit, and lets that area kind of have a bit more room to adjust with baby's kicking/crawling/walking to prevent leaks (at least that's my take on it).
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Re: What are gussets?
i call them poop catchers!
they're the little extra bit of fabric in the leg area of diaper covers. just helps hold in any leaks. they're awesome.