## Definition

**Transaction pinning** is a tactic that uses mempool and relay rules to make another unconfirmed transaction difficult to replace, fee-bump, or confirm quickly. It is most often discussed in Bitcoin-style networks, where transactions wait in the [mempool](/glossary/mempool) before miners choose them for a block.

The pinned transaction is not necessarily invalid. The trick is more subtle: an attacker attaches or arranges related transactions so normal rescue tools run into policy limits. A good mental image is a parcel with a cheap, awkward trailer tied to it. The parcel can still be delivered, but moving it through the queue becomes more expensive and less predictable.

## How It Works

Pinning usually depends on the difference between consensus rules and relay policy. Consensus rules decide what can be included in a valid block. Relay policy decides what ordinary nodes will accept, keep, replace, and forward before mining happens. Transaction pinning lives in that waiting room.

A common pattern involves parent and child transactions. The parent creates an output, and the child spends that output before the parent confirms. This dependency can be useful: [Child Pays For Parent](/glossary/child-pays-for-parent) lets a high-fee child pull a low-fee parent into a block. Pinning turns the same family relationship into friction.

For example, a counterparty may publish a low-fee child transaction that spends from a shared or time-sensitive parent. If someone later tries to replace the parent with a higher-fee version, nodes may reject the replacement because it conflicts with the existing parent-child package, exceeds descendant limits, or fails replacement policy. The attacker has not outbid the honest user; they have made the fee bump harder to relay.

This matters in protocols where timing is part of the security model. Payment channels, swaps, and other pre-signed transaction flows may require one party to confirm a transaction before a deadline. If the other party can pin that transaction during a congested fee market, the honest party may need to pay more, use a different transaction path, or risk missing the window.

## Why It Matters

For miners, transaction pinning affects what profitable work is visible and easy to include. A pinned package may contain a worthwhile transaction, but the attached dependency can distort its effective fee rate or make it harder for mining software to evaluate. Strong [transaction selection](/glossary/transaction-selection) logic looks at related transactions as packages instead of treating every transaction as an island.

For users, pinning is a reminder that "unconfirmed" means negotiable. A transaction can be valid, signed, and widely known while still being hard to accelerate. Fee tools such as [Replace-by-Fee](/glossary/replace-by-fee) and CPFP are powerful, but they are shaped by node policy, package limits, and local mempool views.

For wallet and protocol designers, pinning is a design constraint. Time-sensitive systems need fee-bumping paths that cannot be cheaply jammed by a counterparty. That can mean anchor outputs, package-aware fee logic, shorter dependency chains, or avoiding transaction shapes that leave one party holding a stuck parent with no clean way to raise the [transaction fee](/glossary/transaction-fee).

## Related Terms

- [Mempool](/glossary/mempool)
- [Transaction Fee](/glossary/transaction-fee)
- [Transaction Selection](/glossary/transaction-selection)
- [Child Pays For Parent](/glossary/child-pays-for-parent)
- [Replace-by-Fee](/glossary/replace-by-fee)