Common Misconceptions About Order Execution

Reading time: 3.0-4.0 min

The previous lessons introduced multi-price execution on both sides of the book.

Before moving forward, it is important to correct several intuitive but incorrect mental models that often arise after seeing multi-price execution for the first time.

This interlude does not introduce new rules.
It clarifies how the rules already presented in this tutorial are commonly misunderstood.

Misconception 1 — “A limit price is the price that will be reported”

Why people think this

Limit orders are often described informally as “buying at $X” or “selling at $Y,” which suggests that the limit price is the execution price.

Reality

A limit price constrains whether execution is permitted, not what price is reported.

A trade is recorded only when execution occurs, and the reported price is always the price of the resting order being consumed.

As a result of price-priority execution:

  • a buy order with a limit of $17 may execute at $14 $15 and $16
  • a sell order with a limit of $17 may execute at multiple bid prices
  • the limit price may never appear in the trade tape
  • the reported price reflects executions, not submitted limits

Limits define boundaries, not outcomes.

Misconception 2 — “All liquidity at a price contributes equally to execution”

Why people think this

Liquidity is often described as a total amount available at a price, without regard to execution order or remaining quantities.

Reality

Only executable liquidity contributes to immediate execution.

Executable liquidity consists of:

  • resting orders on the opposite side of the book
  • priced within the incoming order’s limit
  • up to the remaining quantity of that incoming order

Liquidity that exists:

  • at worse prices, or
  • beyond the remaining executable quantity

remains resting, even if it shares a price level with executable orders.

Misconception 3 — “Execution happens at a single price”

Why people think this

Many charts and summaries display only a single price per trade sequence, masking the underlying executions.

Reality

A single order may execute across multiple prices.

When multiple resting orders exist at different prices:

  • execution proceeds by price priority,
  • and continues until the incoming order is fully executed or no further matches remain.
  • Each execution is discrete.
  • Each execution produces a separate trade record.
  • Prices are not averaged or collapsed.

Misconception 4 — “Execution moves continuously through prices”

Why people think this

Multi-price execution often appears smooth or continuous when viewed after the fact.

Reality

Execution is discrete, not continuous.

Even when execution spans several prices:

  • quantities change step by step
  • each trade is a distinct event
  • the trade tape records each execution independently

Execution may feel continuous when several price levels are involved, but it is not.

Misconception 5 — “The reported price represents the best price achieved”

Why people think this

People tend to remember the most favorable price reached during execution rather than the final one.

Reality

The reported price reflects the most recent execution, not the best price reached earlier.

During a multi-price execution:

  • the reported price may rise, then fall
  • the final reported price depends only on the last trade recorded

Price reporting reflects execution order, not best price.

Misconception 6 — “The order book resets between executions”

Why people think this

Tutorials often show isolated snapshots, which can make executions feel as though prior executions do not affect later ones.

Reality

The book is a single persistent object.

Between executions:

  • orders are partially reduced or fully removed
  • arrival times do not change
  • remaining orders persist exactly as recorded

Each book snapshot represents the same book, updated by all events that occurred since the previous snapshot.

Misconception 7 — “Orders act; execution just happens”

Why people think this

Order entry is shown as an explicit step, while the execution mechanism that evaluates and matches orders is often not shown at all.

Reality

Orders do not act.

Orders:

  • define prices and quantities
  • wait passively in the book

Execution:

  • occurs only when compatible orders overlap
  • produces trades
  • updates the book and the tape

Execution is an event, not a property of order entry.

Misconception 8 — “Buy-side and sell-side execution use different rules”

Why people think this

Buy-side examples are more common, and sell-side behavior is often explained separately or incompletely.

Reality

Buy-side and sell-side execution follow the same rules.

The only difference is the direction of price traversal:

  • buy orders execute from the lowest ask upward
  • sell orders execute from the highest bid downward

All other mechanics are identical.

What Always Holds

Across all execution scenarios presented so far, the following rules always hold:

  • Execution is determined by price priority, time priority, and available quantities.
  • Limits constrain execution.
  • Resting orders determine execution prices.
  • The trade tape records only what was actually executed.

Any attempt to explain execution that contradicts these rules will eventually fail.