The author, under appropriate assumptions, extends the theory of scales to $\Pi^2_1$ sets of reals. Scales became central to the descriptive analysis of sets of reals ever since the Axiom of Determinacy (AD) was investigated in the 1960s for its consequences for sets of reals. Loosely speaking, a scale on a set of reals presents the set as a projection of a tree on ordinals with strong continuity conditions. Getting a scale on a set of reals allows one to comprehend and work with it, in the way the classical analytic sets were assimilated. During the 1970s and into the 1980s the extent of having scales assuming AD was studied by the members of the Cabal Seminar well past the projective sets and up to the $\Sigma^2_1$ sets, sets definable not only with quantifiers ranging over the reals but with one more quantifier ``there is a set of reals''. In the early 1980s Martin and Steel showed that in $L({\Bbb R})$, the constructible closure of the reals, $\Sigma^2_1$ sets have $\Sigma^2_1$ scales, and moreover that $\Sigma^2_1$ is the largest class according to quantifier definability that has this scale property. Woodin then showed further that under the assumption of AD${}^+$, a strong form of AD, $\Sigma^2_1$ sets of reals have $\Sigma^2_1$ scales. The next frontier would be the $\Pi^2_1$ sets, the complements of $\Sigma^2_1$ sets, and partial results were established over the years. The author now gets what seems best possible: Let $\Theta$ be the least ordinal that is not a surjective image of the reals and $\theta_0$ the least ordinal that is not a surjective image of reals by an ordinal-definable function. With this, if AD${}^+$ and $\theta_0 < \Theta$, then every $\Pi^2_1$ set of reals has a norms-ordinal-definable scale. The assumptions seem necessary and appropriate, and the conclusion optimal---it is precluded that scales can be any better defined for $\Pi^2_1$ sets. The proof proceeds directly, combining a semiscale construction of Woodin's with a stability property of Jackson's, and thereby bypassing the traditional notion of weak homogeneity.