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Input Sequence (nSequence)

An input sequence, or nSequence, is a field of a Bitcoin transaction input. The sequence of an input was originally intended to enable payment channels similar to the Lightning Network. This design was implemented in the first version of Bitcoin, but such on-chain payment channels were quickly found to be insecure, so the functionality was disabled, leaving the sequence field unused for several years.

With the adoption of BIP 125, the sequence field came into use once again. Currently, input sequences are used to signal support for Replace-by-Fee (RBF), which allow a user to replace an unconfirmed transaction and with a similar transaction that pays a higher fee.

If a user broadcasts a transaction with a low fee, and the transaction is not being confirmed in the desired time, RBF allows the user to increase the fee, incentivizing miners to confirm the transaction faster.

Learn more about Replace-by-Fee (RBF).