AbstractIt is shown that πPhhas a minimum theoretical value ofca.1.7 in aromatic rings. This value is attained whenever phenyl is forced into a highly non‐planar conformation, as may happen throughperi‐interactions or the heavy solvation of sterically hindering neighbouring groups. In the absence of such hindrance πPhwill be greater, and can attain values as high asca.2.6 when there is no steric hindrance to coplanarity and two hetero‐atoms can be shielded. These and a number of intermediate situations are delineated; estimates of πPhare provided for each distinct case. The effect of phenyl substitution atsp3‐carbon is discussed in similar terms. It is shown that these variations make qualitative chemical sense and can be used for predictiv