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.

"Running on All Fours": I did a bit more research on the use of "on all fours" in legal writing, and my research questions whether Michael Quinion is right when he suggests that the phrase refers to two animals standing next to each other (and thus similiarly situated). When I looked up the earliest uses of the phrase in American decisions, it turned out that they all refer to motion — the cases are running on all fours, or traveling on all fours, or going on all fours.

  Here are the four earliest uses of the phrase in state cases, all in contexts designed to mean "closely analogous":

Snelgrove v. Snelgrove, 4 S.C.Eq. 274, S.C., Jun 1812
"[Another precedent] is that most perfectly like the one now before the court. Indeed it may be said to run on all fours."

Gailey v. Beard, 4 Yeates 546, Pa., 1808
"The case in 2 Stra. 934, runs on all-fours with the present"

Abbott v. Broome, 2 Am.Dec. 187, N.Y.Sup., 1803
"The determination also in Saidler and Craig v. Church, goes on all fours with the present case. The facts were exactly similar."

Hamilton v. Buckwalter, 2 Yeates 389, Pa., 1798
"It is similar to the present case, and may be said to run on all-fours."

  And here are the earliest uses in federal decisions:

Russell v. Wiggin, 21 F.Cas. 68, C.C.D.Mass., May Term 1842
"[A prior precedent] is also an authority to the same purpose; and, indeed, it runs on all fours with the present case."

Bank of the U S v. Goddard, 2 F.Cas. 694, C.C.D.Mass., Oct Term 1829
"The case would seem, therefore, to travel on all-fours with the present."

The William Penn, 6 F.Cas. 781, C.C.D.N.J., Oct Term 1819
"[I]t would have been directly in point, and would have gone on all fours with the present."

Anonymous, 1 F.Cas. 1004, D.Md., 1808
"If this determination does not exactly run on all fours with the case to be decided, its principles are so nearly similar as to render an accurate discrimination very difficult."

  Later cases begin to drop the notion of "running" and just say that the former case "is" on all fours with the latter. This is just speculation, of course, but the context suggests that the visual image is more an animal running alongside the observer than two animals standing next to each other. If an animal is running on all four legs beside you, the thinking might be, it means that it remains close to you and goes where you go. So if a precedent runs on all fours with your case, it exactly tracks your case. That's my amateurish guess, at least.