Call resistance & put support
Where the gamma actually sits
A call wall and put support are strikes with the heaviest dealer gamma above and below SPX in the volume book (the chart's default). They can matter because hedging changes near large option concentrations, but they move during the day and are not fixed price targets.
Call Wall and Put Support: Why Price Reacts at Heavy Strikes — 53 seconds, on a real session. Watch on YouTube
Dealer gamma is not spread evenly across strikes — it piles up where the open interest is. The strike carrying the largest concentration above spot is commonly called the call wall (this site labels it call resistance, CR); the heaviest concentration below spot is the put support (PS).
Why price reacts there
Near a heavy strike, dealer hedging intensifies. Approaching a call wall in a long-gamma regime, every further point up forces more dealer selling — rallies run into a mechanical headwind, and in theory price stalls, consolidates or pins near the wall into expiry. Measured on our archive, price holds at a wall no more often than at a random line the same distance away, in every book (what a level hold rate measures). Put support works as the mirror image on the way down. Neither is a hard floor or ceiling: on a strong enough move, or once heavy strikes roll off at expiry, the level simply migrates.
0DTE makes the walls move
With most SPX volume now in same-day options, the walls are not static daily levels — they build and dissolve intraday as flows arrive. That is why FirmTape recomputes them continuously from the tape rather than once per morning: the CR and PS lines on the terminal move as the positions behind them move. Where the walls sit right now, free and fifteen minutes behind, is on /now. The expected-hold band on each session page shows the range those concentrations implied, and the session summary records whether the day respected it; the archive-wide tally is in level hold rates.
Related reading: what GEX measures and the zero-gamma flip.