The Origin of "On All Fours":
One of the legal profession's stranger expressions is that a case is "on all fours" with another case. It means that the former case raises the same facts and legal principles as the latter and is therefore highly relevant as a precedent. You might wonder, what's the origin of the phrase "on all fours"? The answer turns to be kind of interesting.

  According to Michael Quinion's World Wide Words, the phrase "on all fours" originally conjured an image of a four-legged animal like a dog. "On all fours" was originally "on all four," with the word "leg" assumed, so the phrase meant "on all four legs." An animal that walked on all four legs was a strong, stable, and certain animal, as compared to an animal with a bad leg that would have a limp or other unsteady gait.

  How do you get from a phrase meaning strong and stable to a phrase meaning a strong analogy? Here is Quinion's explanation:
In the eighteenth century, people started to use to run on all four as a figurative expression to describe some proposition or circumstance that was fair or equitable, well-founded, sturdily able to stand by itself. To be on all four or to stand on all four meant to be on a level with another, to present an exact analogy or comparison with something else (presumably the image is of two animals standing together, both on all four legs, hence in closely similar situations).
  Now, I don't know if that latter explanation is right. If it is, though, it means that when a lawyer says that one case is "on all fours" with another, he's asking the court to imagine two dogs standing next to each other. Who knew.

Related Posts (on one page):

  1. "Running on All Fours":
  2. The Origin of "On All Fours":