FIX protocol

FIX (Financial Information eXchange) is the standard wire protocol for trading messages between brokers, exchanges, and institutional clients. It's a tag-value text format — 35=D|11=ORDER123|55=AAPL|54=1|38=100|40=2|44=150.00 is a new buy limit order for 100 AAPL at $150 — covering orders, executions, cancels, market data, and settlement. Decades old and essentially universal: if two trading systems need to talk in equities, futures, or FX, they almost certainly speak FIX. Modern derivatives (FIX over WebSocket, FIX binary / SBE) exist for speed, but the original still underpins most of electronic trading.