One could allow 3 conventions 1 a parallelogram is never a trapezoid 2 rectangles are trapezoids but not other paralleograms are 3 all parallelograms are trapezoids I am fairly sure that almost every current US textbook uses convention 1 So that should maybe be noted to avoid confusing students hence I would revert the latest change I personally think 2 makes sense since the main theorem becomes parallel bases equal base angles parallel bases equal legs isosceles trapezoid Also then every quadrilateral in the trapezoid rule is a trapezoid Since b a 0 in this case some of the formulas with b a 2 in a denominator need a fix cancel out the b a 2 from the numerator gentlemath at a public computer