A foot in the door
An introduction or way in to something, made in order that progress may be made later.
The early uses of the term 'putting a foot in the door' are straightforward literal ones. It may just describe someone who steps over the threshold of a property, or someone putting a foot in the door in order to prevent it from closing and so continue a conversation. An early example of the latter comes in the American poet and playwright George Boker's work Plays and poems, 1856:
"And he sang to his gittern of love and of war With one foot in his stirrup and one in her door."
We now use 'foot in the door' in a figurative sense, with a similar meaning to 'the thin end of the wedge'. It was the technique of jamming a foot in the door to prevent it closing, used by door-to-door salesmen and political canvassers, that gave us this figurative use of the term. All the early examples are from the USA, such as in this report of an application for civic funding in The Oakland Tribune, August 1914:
"All I'm asking is that you authorize the park department to go ahead."
"Yes, but you are trying to commit us to an expenditure of $48,400 or more," said Baccus.`
"No. I'm merely asking that the first step be taken," answered Mayor Mott.
"You've got a mighty clever way of getting your foot in the door, and then we can't get it closed until the whole proposition is carried", said Turner.
See other phrases that were coined in the USA.