The basic slotted section measurements of VSWR and the location of the voltage minimum are affected by the variety of errors inherent in the slotted section. Considered here are those effects, important for precision measurements, introduced by discontinuities due to the end of the slot, and due to possible adaptors, coupling elements, bead supports, etc., and by the presence of the slot in creating a guide wavelength and characteristic impedance slightly different from that of a guide without a slot. A calibration procedure is presented which compensates rigorously for these effects in a simple fashion.