Single vs Double Rooms

Single bed means one bed for one person.

Double bed means one bed for two people. (About the size of our full bed.)

A twin is two single beds.

In Germany you will see Einzelzimmer, EZ, which is a room with a bed (twin) for one person. A Doppelzimmer, DZ, is a room for two people, usually with one wider bed, but occasionally with 2 twins. Sometime you will see a Zweibettzimmer, 2BZ, which will have 2 beds, each for one person.

Normally a single room is for one person and a double room is for two persons. The term does not refer to the bed or the bed size; only the number of persons in the room.

A major difference between Europe and the US is that in the US we tend to rent the "room" regardless of the number of people using it but in Europe the rates are almost always dependent on the number of people in the room.

To elaborate, Take Single/Double to mean the number of people, little else. The smaller and older the place, the less standardized the rooms and furnishings will be. I have stayed in "doubles" that have ranged from a single small bed, to a "large" bed for two, to two beds pushed together, a room with three or four beds, and once a two bedroom apartment with a kitchen and living area. If you want something specific, you will need to ask, or just ask to see the room (expected by some hosts).

In Paris "Double" generally means a double bed for two people, and a a "Twin" room is two smaller beds for two people.

When you rent a single you will likely get a small room( and I mean small, I think they sometimes convert old linen closets,, LOL ) with one bed, usally a twin size, but I have on occaison gotten what I think is a double sized bed.
As previous posters say, the older, smaller hotels have lots of character, in other words, rooms come in all shapes and sizes, often no two being exactly alike.