Learn English – Why do Australians and NZers call snacks/lunch *crib*

Twin bed is either of a pair of matching single beds ready to be joined to another twin bed to create twin beds. The word simply describes a bed which is stackable with another bed. A single bed isn't stackable, at least not the way you'd want it to.

UPDATE according to what John Y commented:

Well, wikipedia says:

U.S. terminology refers to a twin bed to mean what is known as a single bed in other countries, whereas other countries understand twin beds to be two single beds in the same room.

While wiktionary says:

twin bed (plural twin beds): Either of a pair of matching single beds.

Oxford dictionary says:

one of a pair of matching single beds, particularly in a hotel or guest room intended for two people.

So to sum it up, and now correct me if I'm wrong:

1 bed (US): twin bed
2 beds (US): twin beds

1 bed (UK): single bed or a twin bed if it belongs to a pair of beds
2 beds (UK): twin bed or twin beds

Therefore twin bed is ambiguous, unless the dictionaries are wrong.

My first guess is "place a call" originated from when the operator would physically have to plug the wires on a switchboard to make a call.

From HowStuffWorks:

Think back to a time when a human being worked in a town's central office. The phone company would build the central office in the middle of town, and then run a pair of copper wires to every home (see How Telephones Work for details on the wiring). The operator -- let's call her Mabel -- would work in the central office. Mabel would sit in front of a switchboard, and on this switchboard would be a collection of sockets -- one socket for each of the phones in town. When you wanted to place a call:

• You would pick up your phone.
• A light above your phone's socket would turn on.
• Mabel would plug a jack into your socket and ask you who you would like to talk to.
• Mabel would then plug her jack into the receiving party's socket, send a ring signal down the line, and talk to the person who answered.
• Mabel would then plug in a wire between your jack and the receiving party's jack to connect the two of you together.
• When she saw the lights go out above your jacks, Mabel would remove the wire connecting the two sockets.

Electrical review, Volume 45 from 1904 talks of placing a call in front of an operator. This corresponds with the switchboard lights turning on in front of Mabel:

Wires should be led from this trunk plug to a desk located outside of the operating room, and having facilities for testing the called line after the plug is inserted, and listening upon it without closing it, so as to place a call before the operator if it is not in use.

So when the caller picks up the phone, they are placing something in front of an operator (some indicating lights) to indicate you wish to make a call. The operator then places some wires in plugs to initiate the call. This gives us at least two senses of things being placed.