Knickers, panties, undies, underpants, Smalls – whatever you want to call them….
Cloth pads really will only work well if the underpants they are in are also doing their job properly!
Snug is good!
I know the temptation is to wear your rattiest old undies when you have your period – so you don’t risk staining your nice pretty ones… but if your underpants are saggy and falling down, they won’t be able to hold a pad securely in place. If the pad isn’t held tight enough against your body, then you’ll have more likelihood of leaks (that’s true for disposables and reusables). They don’t need to be super tight, but by “snug” I mean that they fit comfortably tight enough that they aren’t saggy, they aren’t falling down and the elastic is still doing its job :) If you wear loose fitting styles, such as boxer shorts or “bloomers”, where the fabric is not held snug against the body, then you would most likely need to look at wearing a belted pad.
Natural fibres are good!
Ideally they should be a cotton, cotton-blend or natural fibre. Not just for the breathability, but because a satiny type slippery fabric is going to make the pad more likely to move about than a more non-skid cotton type fabric.
Boyleg, French, Bikini, Granny….?
The type of underpants you wear doesn’t really matter, you should be able to make or buy pads to fit… Even thong/gstring styles. The main issue is the width of the crotch (“gusset”) and the width of your pads. Cloth pads need to snap around the crotch tight enough that they aren’t able to slip around. If the pad snaps too wide for the width of the underpants then it won’t hold firm enough, and may also feel uncomfortable. You can always measure your underpants to help you find the right shape and size pads to suit your needs. Just be aware that sometime the fabric of your underpants will gather up (bunch) while you are wearing them if the crotch is very wide, so measuring the width of the crotch and making sure pads are that wide may end up being too wide for you to comfortably wear.
Ideally your underpants crotch width needs to be wider than the snapped width of the pad you put in there.
If you wear underwear with a narrow crotch then you will need narrow snapping pads, so that the pads snap snugly around the crotch of your underpants. If you wear underpants with a very wide crotch section, you might like to consider using wingless pads, or making a pad system where the wingless pads attach to your underpants (with a snap or slide in under straps like a base+insert pad), so that there are no wings that have to wrap around your underpants. I’ve also seen the suggestion to cut slits in the crotch of the underpants (like making a very long buttonhole) to allow the wings of a pad to poke through the slits and snap around this narrower section, while still allowing the “legs” of the underpants to sit where they would.
If you have underpants with more coverage in the back, like bikini/brief styles, then you should be able to wear pads that have a flared back section. If you wear underpants that have a narrower back section, then you would need to use pads that are also narrow, so they don’t poke out of your underwear.
Bunching and slipping
One thing that seems to come up a lot in cloth pad discussion is “bunching”…. and this has to do with 3 factors. How wide your underpants are, how wide your pad is, and how wide you are (as in your crotch).
Underwear bunching – This is when a pad is snapped around your underpants and it pulls the fabric together at the crotch and sometimes up into the underpants. This is commonly thought to be because the pad is too narrow (in a bad sense), but I don’t think that’s necessarily true. Different underpants have different width crotch, with may be narrower or wider than your own crotch. Underpants fabric is very thin and can easily bunch/gather up there without you knowing in normal wear anyway – as it will scrunch in to fit the space your crotch gives it. If you put a narrow snapping pad there, this bunching is more obvious, but it could simply be pulling the underwear into a width it normally sits at anyway. It doesn’t necessarily mean it’s a bad thing. If the pad is longer, and has wider wings that cover more front/back of the underwear, then they can cause the underpants to bunch/gather up too. Again, this may not be an issue for some, as it’s just some fabric gathered together, but sometimes this may be uncomfortable or undesirable.
Pad bunching – This is where the pad itself “bunches up” or is distorted. Often into a “U” or upside down “U” shape. This can happen if the pad is too wide for your crotch, so the pad squishes to fit the space. This can be uncomfortable for some people and can cause leaks because the pad isn’t held tight against them, while others prefer to have the pad “bunch”, rather than having a pad that sits flat, as there is more surface area for coverage
Slipping – If the pad doesn’t snap tightly enough around your underpants (if the pad width is larger than the width of your underpants), then it may slide around if your underpants are not a tight enough fit, as the wings won’t provide enough friction to keep it in place. If you find your pads are trying to slip off sideways, then the issue may be the wings aren’t snapping tight enough.
See also the pages on Pad Bunching and Pad Slippage
.
Underpants image from http://en.wikipedia.org/wiki/File:Panties_styles_-_en.svg
Reusable Menstrual Products 
