 The apposition of constraint points (z = 0) has the effect, as a rule, of inhibiting every mode which has no set of nodal lines passing through all the aforesaid points, not even with an opportune choice of formula_29.For example, pressing the center of the membrane makes all the modes with formula_30 becoming impracticable, since this point is invariably an antinode for these modes.Pressure on any other point of the membrane (speaking theoretically) makes all the modes with formula_31 practicable, since it will always be possible to have a nodal diameter passing through that point.