What's that thing called?

[Colonel Panic]

My friend just bought bunk beds for her 7 year old. She only bought the main wooden parts and now needs the mattresses and that "thing"

What do you call that metal spring frame thing like the one pictured that you use instead of a box spring?

Found this pic online:



Thanks.

 

[Sad Max]

That's called a linkspring.

 

[Colonel Panic]

Thanks Max!