1. A method for non-invasively measuring blood pressure of a patient, the method comprising: (a) attaching a pressure cuff to an extremity of the patient; (b) attaching a plethysmographic device to the extremity at a distal location relative to the pressure cuff; (c) inflating the cuff to cause occlusion in the extremity and deflating the cuff; (d) determining when the cuff has caused complete occlusion in step (c) by using a device for determining the complete occlusion; (e) taking a series of waveforms representing blood pulsations by using the plethysmographic device; (f) measuring the blood pressure from the waveforms in a processor by multiplying corresponding points in groups of the waveforms in the processor to produce product waveforms and measuring the blood pressure from the product waveforms; and (g) outputting the blood pressure measured in step (f) from the processor.