Every print on the SPX options tape carries an OPRA condition code naming the pipe it came through: electronic book, complex order book, price-improvement auction, a multi-leg order legging into the simple book, or a human being on the trading floor in Chicago. Nobody appears to have published the composition for SPX — the one product where the answer is unusual. So we tabulated five years of it, every code mapped, cancels excluded, zero unmapped residual.
The near book, by year (share of volume):
| channel | 2022 | 2024 | 2026 |
|---|---|---|---|
| electronic simple book | 34.2% | 34.8% | 26.0% |
| complex book + complex auctions | 36.1% | 39.1% | 44.7% |
| physical floor | 20.8% | 19.8% | 20.6% |
| AIM price-improvement auctions | 2.9% | 2.9% | 6.7% |
Three readings:
1. The floor is not dying. A steady fifth of the near book trades by open outcry, five years running — independently matching the one recent academic estimate (Galati, Hendershott, Khan, Riordan: over 20% of U.S. options floor volume, "almost exclusively" S&P 500 index options). The dominant floor code is the proprietary-product multi-leg print: big structures negotiated by humans.
2. The complex book keeps eating the simple book. 36% → 45%, while simple electronic fell to 26%. If your dealer book signs legs individually, nearly half the tape is mis-handled by construction.
3. The auction slice is retail, and it doubled. Cboe's free trade-by-trade sample carries each participant's capacity. Joined to our tape: the initiator of an AIM auction is a Customer 97.8% of the time, the contra is a market maker 95.6% of the time, and 100% of initiating volume is ≤10 contracts — the mechanism's size cap, visible in the data. Small-lot auction internalization is the closest thing SPX has to a printed retail label — and on the 0DTE expiry it reached almost 10% of volume this year.
One more composition fact from the signed book: at the close, dealers end up net LONG gamma on 83% of sessions — the crowd's structural habit is selling same-day optionality into the close.
Full series, channel map and honesty section: What is SPX flow made of?
