9 × 9 boards introduce the longest runs Kakuro allows — up to nine cells, which must contain every digit from 1 to 9 exactly once. Long runs are double-edged: their digit sets are often forced (an eight-cell run's missing digit is just 45 minus the clue), but placing those digits demands patient cross-checking against every crossing clue.
At this size, pencil marks stop being optional. Note the candidates that survive each clue pair, and watch for naked pairs — two cells in a run sharing the same two candidates lock those digits and strip them from the rest of the run.
Every board here, like everything on the site, is verified to have exactly one solution, so a stuck position always has a logical next step. Review the technique guide if a board stalls, or go wider with 9 × 11.