```{eval-rst}
.. _guide-features-chainlocks:
.. meta::
  :title: ChainLocks
  :description: Dash's ChainLock feature uses long-living masternode quorums to expedite transaction security and prevent 51% mining attacks. 
```

# ChainLocks

Dash's [ChainLock](../resources/glossary.md#chainlock) feature leverages [LLMQ](../resources/glossary.md#long-living-masternode-quorum) Signing Requests/Sessions to reduce uncertainty when receiving funds and remove the possibility of 51% mining attacks.

For each block, an LLMQ of a few hundred [masternodes](../resources/glossary.md#masternode) (300-400) is selected and each participating member signs the first [block](../resources/glossary.md#block) that it sees extending the active chain at the current [block height](../resources/glossary.md#block-height). If enough members (at least 240) see the same block as the first block, they will be able to create a [`clsig` message](../reference/p2p-network-instantsend-messages.md#clsig) and propagate it to all [nodes](../resources/glossary.md#node) in the [network](../resources/glossary.md#network).

If a valid [`clsig` message](../reference/p2p-network-instantsend-messages.md#clsig) is received by a node, it must reject all blocks (and any descendants) at the same height that do not match the block specified in the [`clsig` message](../reference/p2p-network-instantsend-messages.md#clsig). This makes the decision on the active chain quick, easy and unambiguous. It also makes reorganizations below this block impossible.

With LLMQ-based [InstantSend](../resources/glossary.md#instantsend), a ChainLock is only attempted once all [transactions](../resources/glossary.md#transaction) in the block are locked via InstantSend. If a block contains unlocked transactions, retroactive InstantSend locks are established prior to a ChainLock.

ChainLocks have been active on the Dash network since block <a href="https://insight.dashevo.org/insight/block/00000000000000112e41e4b3afda8b233b8cc07c532d2eac5de097b68358c43e" target="_blank">1088640</a> in June of 2019. Please read [DIP8 ChainLocks](https://github.com/dashpay/dips/blob/master/dip-0008.md) for additional details.