Atmospheric extinction corrections in astronomical photometry are considered. Most investigators recommend obtaining observations of the object of interest near upper culmination while separate observations proceed on one or two pairs of extinction stars of different intrinsic colors. If the object of interest is a variable star, it may be essential for the observations of it to extend over a large air mass range. A common variable-star differential-photometry technique is to observe a pair of nearby comparison stars whose colors bracket those of the variable. One of the pair is the primary comparison star. A description is presented of a procedure for producing a single variable-star light curve. It is assumed that the primary comparison star is known not to be intrinsically variable. It is important to evaluate the magnitudes and colors of the comparison stars in the standard system.