An all in two nappy is essentially a PUL shell (cover) with separate inserts that normally popper into place in the cover.
The idea with Ai2s is that as long as your baby has only wet the nappy you can simply swap out the insert when changing (and give the cover a quick wipe too if you wish). But this is only possible with a true Ai2, a hybrid (see below) will require the whole nappy to be changed). Of course even with a true Ai2, if your baby has dirtied the nappy you will need to change the whole thing.
As inserts are relatively cheap and covers more expensive this becomes a more economical option because you can use a cover for around 4 times before changing. You can also use any absorbent fabric as an insert (such as muslins, prefolds or even flannels) which will make this a really economical system. Also covers are very quick to dry, whereas inserts take a lot longer so you will definitely need far less covers.
There are two main styles of covers, ones with flaps (sometimes just at the front, sometimes both ends), and those without flaps. All should have one or more poppers to secure inserts (otherwise its basically a wrap not an Ai2). Mot nappies use the same size poppers so you should be able to mix and match inserts. Although you can use any wraps with an alternative insert (such as a prefold), you may find those with flaps more convenient as they will hold the prefold in place better.
I personally don’t find this system suited to younger babies though as they tend to have dirty nappies at every change, but once this stage calms down they are really useful.
Below are a few different types of true Ai2s:
A lot of Ai2 nappies are lined. For me these are not true Ai2 nappies, I call them hybrid Ai2s as they are somewhere between an Ai2 and a pocket nappy.
These hybrids normally have a pocket opening at the back of the nappy so you can add additional inserts. And the poppers on the inside so you can secure a snap-in insert. If you keep the inserts snapped-in (so between the stay-dry liner and your baby) you could, at an absolute push, still just swap out a wet insert at change time if the lining has remained dry, although you won’t be able to wipe down the cover. But this is something I’d do more for emergencies, and its also fairly unlikely that the lining will have remained dry unless you baby has only done a small small wee. If you have put the inserts into the pocket instead then you will definitely need to change the whole time at change time, just like a normal pocket nappy.
Below are a few different types of hybrid Ai2s:
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