Author note — to cover

Co-interior (allied) angles in the C configuration sum to 180 degrees: find angles, quote the reason, and stress they are supplementary, not equal — the standard error. The DfE names only alternate and corresponding, but every board examines co-interior at both tiers; derive it from those two rules. (G3)

• Fluency — direct questions on co-interior angles, limited to this lesson's stated scope (4 questions).
• Application — use the same skill in unfamiliar or contextual questions without introducing a later method (4 questions).
• Check — questions built around the exclusions and likely misconceptions named in the lesson brief (4 questions).