Dallan's law

Dallan's law is an epigram that is typically stated as: "If angle looks like 90 degrees, then it is 90 degrees."

The law is often followed by mistake in math where someone assumes that an angle in a triangle is 90 degrees so they can use rules that only can be applied to right triangles.

"90 degrees" can sometimes be a metaphor for something else e.g., "If someone sees a car as red, then it is red". This means that a thing can be perceived as something else even though it is obvious that it only can be one thing. A washing machine can therefore be perceived as a rocket launcher if that person is stupid enough.

Example 1:

David: Why do I get the wrong answer when I use the Pythagorean theorem?

Teacher: Beacause it does not contain a right triangle. You probably utilized Dallan's law.

Example 2:

David: What a nice avocado you got there!

Åsa: Thanks David, but that's a potato. I guess you're right though according to Dallan's law.