Double-headed nail, USA: "This object is called a double-headed nail — a nail with two heads. Construction workers in the US use them to install temporary wooden supports during building. The second head of the nail always protrudes, so that when the temporary supports need to be dismantled, the second head allows the nail to be easily removed. It's a striking example of locality, when what is normal to one country can be so uniquely original to another. For a lifetime you might think that nails only come with one head."