Inordertomeasure a circlethough, theydomaintain a centralpointforreferenceaswellasanimaginarylinethatextendsfromthecentertotheedgestogivethelengthoftheradiusandthediameter.
Thisline, althoughimaginarytotheconstructof a circle, iscrucialbecausewithoutitcircleswouldn't exist.