planes of rotation
... that a double rotation in four dimensions has exactly two planes of rotation?
... that a <a class="mw-redirect" href="./SO(4)#Double_rotations" id="mwEdM" rel="mw:WikiLink" title="SO(4)">double rotation</a> in <a href="./Four-dimensional_space" id="mwEdQ" rel="mw:WikiLink" title="Four-dimensional space">four dimensions</a> has exactly two <b id="mwEdU"><a href="./Plane_of_rotation" id="mwEdY" rel="mw:WikiLink" title="Plane of rotation">planes of rotation</a></b>?