Just wondered, but why are the knickers built in to the women's dresses ? Is that the same for all dancers (outside of Strictly in normal places), or only on the show ?

Just seemed a bit weird to me as long ballgowns, for instance, are not exactly going to show anything rude are they. Must be a pain to go to the loo.

They are built in for normal competition dresses and, believe me, it is very inconvenient since you have to stand around some time in them waiting for your round and then, if you get to the final, the results. I have a convertible Latin/Ballroom dress where the ballroom skirt fits over the Latin dress, and have to allow a fair amount of time if I need to go to the Ladies. My latest ballroom dress has popper openings in the gusset, but that would be a bit risk for a Latin one, if the fastenings came apart.

Anyway, the reason is to provide as smooth a silhouette as possible. Often the back of the dress is quite low, and it has fitted bra cups, and so a leotard type body gives the smoothest finish with no riding up. The lycra is usually quite firm and so gives a bit more control round the stomach too.