# Parallel and Perpendicular axis theorems

#### maithly

Could someone explain me perpendicular and parallel axis ????

#### MBW

Your post is a bit vague, so I'm not sure if I am actually answering the question you are asking.

An axis is simply a direction of measurement, chosen fairly arbitrarily according to what is convenient for the given situation.
For the room I am sitting in, it would probably be sensible to chose to measure along the corners, starting perhaps at the bottom corner nearest the door, the computer is then 2 meters along 1 meter in and half a meter up (it is a small room).
Those were perpendicular axes, each at 90 degrees to the other, (and each measurement was parallel to one of the walls).
I could have chosen axes which went diagonally and which aren't at 90 degrees, but the measurements would (in general) be more awkward, thus perpendicular axes are usually used because they are easier to visualise and work with.
I could also have chosen to start my axes at the top of the wall above the window, these axes would then be parallel to the set defined from the bottom of the door.
There is also the parallel axis theorem, which is used when determining the moment of inertia of an object.

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