What is Timelock?

Timelock is the condition on the blockchain that a transaction must meet to be processed at a certain predetermined time or block height. The concept was initially introduced by Satoshi Nakamoto in the whitepaper of Bitcoin. The timelock mechanism is used to schedule transactions for the miners to process them at a specified time.

Transactions are added to the blockchain as blocks located at certain heights. Once the transaction data is added to the chain, the transaction is deemed approved. 

The timelock mechanic is used to ensure that transactions are not verified unless their predetermined conditions, i.e.,  a certain time or block height, are reached. Time time is measured using the Unix system.

There are two types of timelocks:

• Absolute timelocks – they discern that the blocks are defined relative to a specific time;
• Time-relative timelocks – they are used to determine how much time has to pass until a transaction is verified.

Bitcoin transactions measure time in two ways, based on the block number and its timestamp. If the timelock is determined according to the block number, the miners receive the specific number for the validation process. If the timelock is based on a timestamp, the miners must wait until the given point in time to verify the transaction.

There are four methods for determining the Bitcoin timelocks which occur on either the transaction level or the script level:

• nSequence – absolute time blocking at the transaction level;
• nLocktime – timelock relative to the transaction level;
• CheckLockTimeVerify – absolute time blocking at the script level;
• CheckSquenceVerify – timelock relative to the script level.