The "protocenter", defined as an atom carrying two different attached groups in a nonlinear arrangement, is proposed as a concept useful for the introduction of chirality and geometric isomerism in introductory organic chemistry classes. Two protocenters are the minimum requirement for stereoisomers of a compound to exist. Protocenters may be coaxial or noncoaxial; for coaxial protocenters, depending on the location (coincident or noncoincident) and orientation (coplanar or noncoplanar), the outcome may be enantiomers, geometric isomers, or an achiral compound. The use of the concept in determining the kind of possible stereoisomerism for compounds is illustrated using examples representing a range of differing structural types. (Contains 3 tables and 1 figure.)