Skill: Subtract one less

Subtracting 1 from a whole number produces the number immediately before it in the counting sequence: $n-1$ is one less than $n$, including cases such as $10-1=9$ and $1-1=0$. A number line or ten-frame can show this one-step decrease for numbers typically within 20, distinguishing it from subtracting a larger amount or finding one more; this understanding supports efficient subtraction and place-value reasoning.