The strike with the heaviest call open interest. As price approaches it, dealer hedging (buying stock to stay covered on short calls) can turn into resistance right around that level.
Use it as: a probable ceiling for the day. Fading a push into it (short call spread, or a stop-guarded short) uses the wall itself as your invalidation — a clean close through it means the ceiling is gone, not just tested.

The mirror image — heaviest put open interest, acting as a probable floor via the same hedging mechanism in reverse.
Use it as: a bounce/long candidate on a push down into it, wall as your stop reference. Call Wall and Put Wall together define your expected range for the session.

The one PURE FACT here (no dealer-positioning assumption needed) — the strike where option holders collectively lose the most if price settled there today. There's a real, if soft, tendency for 0DTE price to drift toward this level as expiration nears.
Use it as: a late-session tiebreaker, not a morning target. Mostly ignore it early in the day; in the final 60-90 minutes of a 0DTE expiry, it's a mild pull worth weighing if you're holding a position moving away from it.

The price where AGGREGATE dealer gamma crosses from negative to positive. This sets the character of the whole day, not just a level.
Above the flip (positive GEX): hedging dampens moves — choppy, mean-reverting, breakouts tend to fail, fades tend to work.
Below the flip (negative GEX): hedging amplifies moves — trendier, more volatile, breakdowns can accelerate, dip-buying is riskier than usual.
Use it as: the first thing to check, before picking a level to trade off — it tells you whether you're in fade-the-extremes conditions or moves-can-run conditions. Matters most when spot sits close to it — a confirmed break of the flip itself is a real regime-change signal.

1. Check Gamma Flip vs spot first — decide if today is a fade day or a trend day.
2. Use Call Wall / Put Wall as your range — levels to fade (positive GEX) or watch for a confirmed break through (negative GEX).
3. Use Max Pain as a late-day tiebreaker only, not an early trigger.
4. Your stop is almost always the wall itself — a clean close through it invalidates the fade.

Reminder: this is how the mechanics translate into a thesis, not a signal to follow blindly. Position sizing, your own risk tolerance, and everything else happening in the tape still sit on top of all of this. Not financial advice.