The default route to the tonic: 2-⁷ → 5⁷ → 1, or just 5⁷ → 1 — powered by the leading tone rising a half step and the dominant's internal tritone collapsing onto the target. It's the resolution the ear predicts before it happens, which makes it both the strongest cadence available and the baseline every other entrance is named against — the term comes from jazz pedagogy, where the backdoor was coined by contrast with it. Use the front door when you want certainty delivered; use the other doors when you want that certainty to be the thing you play with.