Would need to be -17 to be parallel to the ground and top tube.
Not true. It depends on the head tube angle. It just happens that most have a head tube angle of about 73 degrees. 73 + 17 = 90 = parallel. A steep head tube at 80* degrees would need a -10 degree stem to be parallel to the ground.
*for example.
@Pete4Eyes started
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Would need to be -17 to be parallel to the ground and top tube.