Unintuituve naming convention: "para-" denotes antiparallell? I did a project a week ago that required studies of the helium atom. Depending on if the two electrons spin are parallell or anti-parallell it is referred to as orthohelium (parallell) or parahelium (antiparallell). Intuitively I would have imagined the naming convention the other way round: parahelium for parallell spins. Today I "corrected" a professor in subatomic physics when positronium came up which apparently has the same naming convention. My thoughts are along the line: "if even HE finds it unintuitive, why is it so?" :-) Is it something else that is parallell/orthogonal, or is it just historic reasons? Google didn't help me.