GEOMETRY
posted by Twg on .
The legs of an ironing board are equal in length and
bisect each other at the point where they cross.
What generalization about
parallelograms ensures that the ironing board will
always be parallel to the floor, regardless of the height
of the board above the floor?

What does "parallel" mean?

level to doesn't it?

A parallelogram has two sides that are the same distance apart. They are parallel  like this:
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Here's a site about parallelograms.
http://www.ies.co.jp/math/products/geo1/applets/para/para.html
I'm sure you'll be able to answer the question now. Please post your answer, and I'll be glad to check it. 
ok, is it because the legs are the same size??

Your question is: "What generalization about parallelograms ensures that the ironing board will always be parallel to the floor, regardless of the height
of the board above the floor?"
This part of the above site answer the question.
1. The opposite sides are equal in length.
. . .
3. The diagonals bisect each other. 
so the opposite legs are the same in length even though they cross each other. That's the answer?

The legs are diagonals of the parallelogram, and they bisect each other.