The major scale wearing its mode name — the yardstick every other card measures against. Its two half steps sit at 3–4 and 7–1, and that's where all the gravity lives: the 7 leans up into home, the 4 leans down onto 3, and the 5⁷ chord contains both, which is why 5 → 1 feels like a door closing. Every diatonic progression on this site defaults to this world. The whole mode game is moving those two half steps somewhere else and hearing what changes.
The natural 7 — the leading tone, a half step under home; the pull that makes 5 → 1 feel like gravity.
It IS the parent — every other card on this shelf borrows this exact pitch set and moves the front door.
'Happy' undersells it — Ionian is the mode of resolution, and songs that never leave it can flatline. That's what borrowing is for: visit a darker mode's chord so that coming home means something.
Everywhere — 'Ode to Joy,' 'Let It Be,' most of the songbook on this site. It's the water the other six modes swim against.