Maths question?

There is no convention...landscape vs portrait...

generally, length is the longer dimension...

What do you have to calculate ? Give an example...

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Once in the fifth grade, I was called to the blackboard to solve a problem in front of the class. I deliberately labeled the short side as length, but had to change it to width when the teacher "corrected" me. Back then, I accepted what I was told without question.

I have since learned that teachers make mistakes and have biases—as does everyone. Most people make label the long side as length, but that is not a law. You can label the short side as the length, just as you can draw a map with north at the bottom.

However, you run the risk of having your work marked as incorrect. Then you would have to argue to get full credit, and the teacher might claim "I've told the class to always use 'length' for the long side. You must not have been listening."

The length of a computer screen is usually its width, except on a Kindle where the length is the height.

In maths, there is no length, height or depth. They are simply names which we give to the dimensions of an object so that we find it easier to imagine them. They could just as easily be called 'a', 'b' and 'c'.