Level 9 · Advanced technical layer

Timelocks: nLockTime, CLTV, CSV and sequence

The difference between delaying a transaction and making a UTXO spendable only after an absolute or relative time.

Article: 87
Reading time: 14 minutes
Reviewed: 10 September 2026

In a nutshell

nLockTime limits when a transaction may be included. CHECKLOCKTIMEVERIFY puts an absolute threshold in an output condition, while BIP 68 and CHECKSEQUENCEVERIFY create a relative delay measured from the spent UTXO's confirmation.

An absolute transaction lock

nLockTime sets the earliest height or time at which a transaction may enter a block, subject to its input sequence values. It does not stop a key holder from signing a different transaction unless the output script itself requires a lock.

CLTV locks a script branch

BIP 65 makes OP_CHECKLOCKTIMEVERIFY compare a script value with the spending transaction's nLockTime. A policy can allow joint spending now and a recovery key only after a threshold, but every alternative branch must be reviewed.

CSV measures a UTXO's age

BIP 68 gives sequence fields relative-lock meaning in version 2 or later transactions. BIP 112 lets a script require that age with CHECKSEQUENCEVERIFY, a building block used by Lightning penalty delays.

It is not an alarm clock

Blocks arrive randomly and time-based rules use median-time-past rather than one computer's clock. A timelock neither broadcasts a transaction nor provides a signature or fee; somebody must still produce a valid spend after the threshold.

Level 9 · Advanced technical layer

Terms to know

Absolute lock: A threshold tied to a chain height or time.
Relative lock: A delay measured from confirmation of the output being spent.
Median-time-past: A median of recent block times used by consensus time rules.

Common misconception

A timelock automatically sends bitcoin on a chosen date.

A more accurate explanation

It only defines when a transaction or branch may become valid; the spend still needs signatures, broadcast and a sufficient fee.

A more accurate explanation

Can miners skip the wait by changing a clock?

Timestamps have consensus bounds and timelocks use median-time-past. Miners have limited discretion, not arbitrary power to bypass the rule.

Key takeaways

01nLockTime restricts one transaction.
02CLTV puts an absolute threshold in Script.
03CSV requires a relative UTXO age.
04Block time is not exact calendar time.

A child-friendly recap

In very simple terms

A timelock means “wait first.” It never sends bitcoin by itself; after the chosen block or time, somebody still has to create and publish the valid transaction.

Reviewed: 10 September 2026