You might hear gussets being talked about and wonder what on earth the difference is. Most nappies have a single gusset, i.e. there is the normal leg elastic for containment and fit but nothing else. If you want to explore leg elastics in more detail, then take a look at the leg elastics page.
Some nappies feature a double gusset. This gives an extra barrier of protection, not quite as good as a two-parter system would but better than with just a single gusset.
There are two options for the design of a double gusset, either exterior or interior.
An exterior gusset is a little more common. Essentially it’s an extra frill of fabric and elastic positioned just outside the leg elastics. This will help with any escaping solids. the downside is it can often leave sock marks, although I haven’t found this to be the case with all brands.
An interior gusset is an additional line of elastic inside the nappy. It is either created by sewing a gathered line into the lining material. Or by adding elastic to the edges of the absorbent layer. As well as capturing solids they can also prevent ‘flooding’ which is when older children hold their bladder for a while and then produce a sudden, large amount of urine. The downside is, as well as additional sock marks it can also make solids more difficult to remove as it can gather in the elasticated creases.
I would say double gussets are particularly useful on younger babies, especially before weaning when even solids are something of a liquid, and you might struggle to get a good fit on tiny legs. But I personally don’t see their value as much on older children. I’m also not convinced of the value of an interior gusset for older children either. The easiest way to combat flooding is to use an insert that instantly absorbs fluid, and the best material for this is cotton. But hopefully this has been a useful explanation between the differences.