nSequence

nSequence is a 32-bit field per transaction input in Bitcoin. Originally intended as a partial-replacement mechanism in Bitcoin's earliest design, it has since been repurposed by BIP-68 (and signaling for BIP-125 RBF) to encode:

• Relative locktimes. Lower 16 bits encode a block-count or time-based delay since the input's UTXO was confirmed.
• Type flag. Bit 22 chooses between block-based and time-based interpretation.
• Disable flag. Bit 31, when set, disables relative-locktime enforcement for that input.
• RBF signaling. Any nSequence value less than 0xfffffffe signals BIP-125 opt-in RBF.

The current default value is 0xfffffffd (BIP-125 RBF signaling enabled) or 0xffffffff (no relative locktime, no RBF signaling), depending on the wallet.

The semantics matter for a few cases users actually encounter:

• A wallet that supports RBF sets nSequence to 0xfffffffd so the transaction can be replaced via BIP-125.
• A Lightning commitment transaction uses specific nSequence values to encode the channel-close delay window enforced via OP_CHECKSEQUENCEVERIFY.
• A scripted contract with relative locktime sets nSequence to the delay value and pairs the input with a CSV check in the locking script.

nSequence is one of those Bitcoin protocol details that most users will never encounter directly, but that's quietly load-bearing for Lightning, vaults, and most advanced multi-party constructions. The history of repurposing it from "partial replacement" to "relative locktime + RBF signaling" is a small example of how Bitcoin's protocol pragmatically reuses field semantics over time.

See BIP-68 for the relative-locktime semantics and Locktime for the broader time-based scripting framework.