The linear algebraic features of foveal colorimetry do not apply to 10-degrees degree colorimetric fields. A light presented as a foveal 2-degrees field stimulates the three classes of cone photoreceptors, and a light presented as a 10-degrees field stimulates the cone and rod photoreceptors. While no more than three primary lights are required to match either of these lights, large-field color matches with three primaries do not obey Grassmann＇s laws of linearity because the relative contribution of the rod signal to a color match changes with retinal illuminance. A three-primary set of color-matching functions (CMFs) affected by rod-intrusion, therefore, cannot serve as the basis for colorimetry. Stiles and Burch removed the effects of rod activity from their 10-degrees color-matching data to create a set of CMFs that have the same linear algebraic features as foveal, 2-degrees colorimetry. They did this by selecting the primaries appropriately, making the matches at high retinal illuminance levels, and removing the small residual effects with numerical techniques. The resulting CMFs formed the major portion of the CIE 1964 10-degrees Standard Observer. These CMFs describe color matches at the level of the cone-photoreceptor and, therefore, can be generalized to predict color matches with any set of primaries, but they cannot account for the effects of rod activity in the matching sample. If two colorimetric fields differ in the amount of rod activity they produce, calculations with the 1964 Standard Observer may predict their metamerism inaccurately. This article summarizes Stiles and Burch＇s methods for removing the effect of rods, gives examples of the implications for colorimetry, and suggests procedures for preventing the adverse effects of rod activity in applied situations. It also shows that three recently published sets of 10-degrees CMFs may be influenced by the effects of rod activity.