Basic solutions get their own p-number: pOH counts the hydroxide side, $[\mathrm{OH^-}]$, exactly the way pH counts $[\mathrm{H^+}]$. In any water solution at room temperature the two ends are chained together: $\mathrm{pH} + \mathrm{pOH} = 14$ — read aloud: pH plus pOH equals fourteen. Learn one end and the other comes free — the cleanest mental arithmetic in chemistry.

Picture one ruler read from both ends: a solution at pH 11 sits at pOH 3, and 11 + 3 = 14, always. The see-saw SUBTRACTS from 14 — it never echoes your number back, and it never adds on top of 14. (The 14 is water's own equilibrium constant in disguise, pK-w — a story grade 12 finishes.)