Airflow through the nose is a complex function since the nose is a tortuous series of channels. Mathematicians can only guess at formulas which would explain the intricacies of the nasal passages and flow of air through them. For practical purposes a "black box technique" may be used in which input and output are measured at either end of the black box (the nose). A very useful and accurate measure of nasal patency is obtained from these measurements. Rhinometry has been limited in research environment but is now feasible for clinical practice. With the advent of simpler techniques and improved instruments any physician can be trained to do rhinometry.