Definition of "dos"

With the coming of Monday arrived a new life for David—a curious life full of "don'ts" and "dos." David wondered sometimes why all the pleasant things were "don'ts" and all the unpleasant ones "dos."

1916, Eleanor H. Porter, chapter VIII, in Just David‎

In functional harmonic progression, three “Dos” in a row within the Do-Ti test indicate chord changes that can only be this descending third pattern.

2020, Jennifer Snodgrass, Teaching Music Theory, Oxford University Press, page 212