Faulds and tonlets are both pieces of armor worn below the breastplate to protect the groin and waist, but some major differences separate them.
FAULD + TASSETS
Worn over a skirt of maille, a fauld is a piece of armor that sits under a breastplate, corresponding to a ‘culet’ which sits under the backplate on the other side (though in early to mid medieval times, culets were rarely worn as maille over the arse was deemed ‘good enough’). A fauld consists of steel lames connected by strips of leather, albeit other less popular methods were sometimes used to connect them. The leather strips allow expansion and contraction, which is absolutely necessary because the fauld needs to contract when the wearer mounts a horse. Oftimes, additional ‘tassets’ were hinged to the bottom of faulds to hang over the ‘cuisses’ and provide additional protection for thighs.
A tonlet is very similar to a fauld, but it’s more difficult to be comfortable on a horse while wearing one. You may need a hand mounting. This is because tonlets are thrice as long and don’t have as much flexibility, though by looking at many images you can see how they would be able to contract somewhat, making jousting possible. Their purpose is to be rigid, supplying great if not desirable protection against lances or even lead balls. There’s no use attaching tassets to a tonlet because they already reach down close to the knees, depending on the particular size as many were built specially. Tonlets wrap around the entire waist like a skirt, so there’s also no need for a culet. They supply much more protection than faulds, as you can imagine, perfect against the deadlier weapons of the time, like crow’s beaks.
Tonlets were occasionally worn by nobles in tournaments. Faulds, on the other hand, were meant for open battle, meant to allow near-perfect maneuverability for the common knecht. With all that said, I’d much rather wear a tonlet behind a bulwark; thank you very much!