What is Diff. btwn Double, and double-twin?
 

And can 2 ppl saty in a single?? This is baffling me. Specifically, I am referring to Paris hotels, but likely is the same elsewhere?!

A double is a room with a double or larger bed. A double-twin is a double room with two twin beds. No, 2 people can't stay in a single.

A double would have a bed that is for 2 people, a "double" bed; double-twin would have 2 twins. Two people cannot stay in a single.

As logic would have it, it makes sense--but i easily confuse myself!
Thanks all!

Two can't stay in a 'single' but one can stay in a 'double' :)
(in case you see a rate for "Double - single use")

Two people can hardly share a double bed. Remember a double bed is exactly the width of two crib mattresses side by side--but longer, of course. If I can't determine if queen or king is available I just request a twin bedded room. I inquired about the size of a bed in Italy and was told it was 4 inches less in width of a double mattress. It was their only available room but I had to decline. If you ever hear about the "matrimonial" size bed it is a king size.

No - two people cannot stay in a single room - unless one is willing to hide - and sleep in the bathtub. A single room has one single bed (like you had as a child)

Lots of single rooms do have double beds, however they are very small and I believe when a hotel labels a room a single, they won't put two people in those rooms. You would not want to, in any case, and if they did, they would charge you a double room rate because there would be two people in it.