The definitions
The Call Wall is the strike above the current price, within about 3% of it, with the largest call gamma in the chain. The Put Wall is the strike below the current price, within the same window, with the largest put gamma. Both are computed from open interest across all expirations and recomputed as price and positioning move. The window matters: a strike 5% away with enormous long-dated open interest is a hedge, not a level the day can reach, so it is shown separately rather than as the wall.
A wall is not a line someone drew: it is a place where a lot of options are open, and therefore a place where dealers will have to trade a lot of the underlying if price gets there.
Why they behave like resistance and support
As price approaches the Call Wall, dealers who are long those calls' gamma sell into the rise to stay hedged, and the calls' pinning pull grows as expiration nears. The result is a level that is hard to get through and easy to get stuck at, which is why the Call Wall so often marks the high of the day or of the week.
The Put Wall works the same way in the other direction: dealer hedging around a heavy put strike absorbs selling, so it tends to mark the low. In a negative gamma regime the walls are weaker, because dealer hedging there is no longer leaning against the move, and that is exactly when breaks are more common.
How the two differ
The Call Wall is a magnet and a ceiling: price often grinds up to it and stalls. The Put Wall is more of a floor: price tends to reach it fast, in a flush, and either bounce or break. When the Put Wall gives way with the index already below Zero Gamma, the next level down matters a lot more than usual.
The distance between the two walls is the range the options market is positioned for. A wide gap says the market expects room to move; a narrow one says it expects a pin.
How traders use them
As the frame of the day. Where are the walls relative to the open, how much room is there to each, and which one is price heading to? A move that reaches a wall and holds is a place to reduce risk or fade; a move that breaks a wall and holds beyond it is a change of context, and the next wall becomes the target.
On expiration days the Max Gamma strike and the walls become pins: the closer to the bell, the harder price finds it to leave a heavy strike.
How often do they hold?
GexVision measures it every session: a wall counts as touched when price comes within 0.15% of it, and as held when the close stays on the inside. The last 30 days by symbol, plus yesterday's session with the events in order, are on the public levels pages: SPX, NDX, SPY, QQQ. We publish the numbers rather than adjectives.