Maker-Taker

Makers (liquidity providers who place resting limit orders) earn a rebate per share

Takers (liquidity consumers who execute against resting orders) pay a fee per share

Maker-Taker Fee Flow:

MAKER posts limit order
    └→ Order rests in book (adds liquidity)
        └→ TAKER executes market order against it
            ├→ Maker EARNS rebate:  $0.0020/share
            ├→ Taker PAYS fee:      $0.0030/share
            └→ ECN KEEPS spread:    $0.0010/share

The rebate incentivizes tighter spreads → more volume → more rebate opportunities → tighter spreads. A self-reinforcing flywheel.