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## Understand nSequence
Every UTXO used in a transaction has an `nSequence` (or if you prefer `sequence`) value. It's been a prime tool for Bitcoin expansions as discussed previously in [§5.2: Resending a Transaction with RBF](5_2_Resending_a_Transaction_with_RBF.md) and [§6.4 Sending a Transaction with a Locktime.md](6_4_Sending_a_Transaction_with_a_Locktime.md), where it was used to signal RBF and `nLockTime`, respectively. However, there's one more use `nSequence`, which was described by [BIP 68](https://github.com/bitcoin/bips/blob/master/bip-0068.mediawiki): you can use it to create a relative timelock on a transaction.
Every UTXO used in a transaction has an `nSequence` (or if you prefer `sequence`) value. It's been a prime tool for Bitcoin expansions as discussed previously in [§5.2: Resending a Transaction with RBF](5_2_Resending_a_Transaction_with_RBF.md) and [§6.4 Sending a Transaction with a Locktime.md](6_4_Sending_a_Transaction_with_a_Locktime.md), where it was used to signal RBF and `nLockTime`, respectively. However, there's one more use for `nSequence`, described by [BIP 68](https://github.com/bitcoin/bips/blob/master/bip-0068.mediawiki): you can use it to create a relative timelock on a transaction.
A relative timelock is a lock that's placed on a specific input of a transaction and that's calculated in relation to the mining date of that transaction. For example, if a UTXO was mined at block #468260 and a transaction was created where the input for that UTXO was given an `nSequence` of 100, then the new transaction could not be mined until at least block #468360.
A relative timelock is a lock that's placed on a specific input of a transaction and that's calculated in relation to the mining date of that UTXO. For example, if a UTXO was mined at block #468260 and a transaction was created where the input for that UTXO was given an `nSequence` of 100, then the new transaction could not be mined until at least block #468360.
Easy!
> **WARNING:** `nSequence` is only interpreted as a relative timelock if the transaction is marked with `nVersion` of 2 or more. This is default for Bitcoin Core starting with version 0.14.0.
> **SEQUENCE NOTE (III):** This is the third use of the `nSequence` value in Bitcoin. Any `nSequence` value from 1 to 0xf0000000-1 will be interpreted as a relative timelock if `nVersion ≥ 2`. You should be careful to ensure that relative timelocks don't conflict with the other two uses of `nSequence`, for signalling `nTimeLock` and RBF. `nTimeLock` usually sets a value of 0xffffffff-1, where only `nTimeLock` is allowed of the three; and RBF usually sets a value of "1", where RBF and `nTimeLock` are both allowed and where a relative timelock is irrelevent, because it defines a timelock of 1 block. In general, remember: a `nVersion` value of 2 and a `nSequence` value of 0x00000001 to 0xf0000000-1 allows relative timelock, RBF, and `nTimeLock`; a `nSequence` value of 0xf0000000 to 0xffffffff-2 allows RBF and `nTimeLock`; a `nSequence` value of 0xffffffff-1 allows only `nTimeLock`; a `nSequence` value of 0xffffffff allows none; and `nVersion` can be set to 1 to disallow relative timelocks for any value of `nSequence`. Whew!
> **SEQUENCE NOTE (III):** This is the third use of the `nSequence` value in Bitcoin. Any `nSequence` value from 1 to 0xf0000000-1 will be interpreted as a relative timelock if `nVersion ≥ 2` (which was the default starting in Bitcoin Core 0.14.0). You should be careful to ensure that relative timelocks don't conflict with the other two uses of `nSequence`, for signalling `nTimeLock` and RBF. `nTimeLock` usually sets a value of 0xffffffff-1, where a relative timelock is disallowed; and RBF usually sets a value of "1", where a relative timelock is irrelevent, because it defines a timelock of 1 block. In general, remember: with a `nVersion` value of 2, a `nSequence` value of 0x00000001 to 0xf0000000-1 allows relative timelock, RBF, and `nTimeLock`; a `nSequence` value of 0xf0000000 to 0xffffffff-2 allows RBF and `nTimeLock`; a `nSequence` value of 0xffffffff-1 allows only `nTimeLock`; a `nSequence` value of 0xffffffff allows none; and `nVersion` can be set to 1 to disallow relative timelocks for any value of `nSequence`. Whew!
### Create a CSV Relative Block Time
The format for using `nSequence` to represent relative time locks is defined in [BIP 68](https://github.com/bitcoin/bips/blob/master/bip-0068.mediawiki) and is slightly more complex than just inputting a number, like you did for `nTimeLock`. Instead, the BIP sepecification breaks up the four byte number into three parts:
The format for using `nSequence` to represent relative time locks is defined in [BIP 68](https://github.com/bitcoin/bips/blob/master/bip-0068.mediawiki) and is slightly more complex than just inputting a number, like you did for `nTimeLock`. Instead, the BIP specifications breaks up the four byte number into three parts:
* The first two bytes are used to specify a relative locktime.
* The 23rd bit is used to signal if the lock refers to a time rather than a blockheight.
* The 32nd bit is used to signal if relative timelocks are deactivated.
If you want to create a block-based relative timelock, the construction is still quite easy
If you want to create a block-based relative timelock, the construction is still quite easy: you set `nSequence` to a value between 1 and 0xffff (65535). The new transaction can be mined that numebr of blocks after the associated UTXO was mined.
### Create a CSV Relative Time
[WARNING: NOT REALLY USED]
You can instead set `nSequence` as a relative time, where the lock lasts for 512 seconds time the value of `nSequence`.
In order to do that:
1. Decide how far in the future to set your relative timelock.
2. Convert that to seconds.
3. Divide by 512.
4. Round that value up or down and set it as `nSequence`.
5. Set bit 23 to true.
To set a time 6 months n the future, you would first calculate as follows:
```
$ seconds=$((6*30*24*60*60))
$ nvalue=$(($seconds/512))
$ hexvalue=$(printf '%x\n' $nvalue)
$ relativevalue=$(printf '%x\n' $((0x$hexvalue | 0x400000)))
$ echo $relativevalue
4224679
```
If you convert that back you'll see that 4224679 = 10000000111011010100111. The 23rd digit is set to a "1" and the first 2 bytes, 0111011010100111, convert to 76A7 in hex or 30375 in decimal. Multiply that by 512 and you get 15.55 million seconds, which indeed is 180 days.
## Create a Transaction with a Relative Timelock
So you want to create a relative timelock? All you have to do is issue a tranaction where the `nSequence` in an input is set in accordance with the above: with the `nSequence` for that input set such that the first two bytes define the timelock, the 23rd bit defines the type of timelock, and the 32nd bit is set to false. It's a straight up two-byte number for a timelock based on relative blockheight and it's a two-byte number `0x400000` for a timelock based on relative time.
Issue the transaction, but it can't legally be mined until enough blocks or enough time has passed beyond the time that the transaction forming the UTXO was mined.
Except pretty much no one does this. The new [BIP 68](https://github.com/bitcoin/bips/blob/master/bip-0068.mediawiki) definitions for `nSequence` were incorporated into Bitcoin Core at the same time as [BIP 112](https://github.com/bitcoin/bips/blob/master/bip-0112.mediawiki) which describes the CSV opcode, which works with `nSequence`, just like the CLTV opcode works with `nTimeLock`. Just like CLTV, CSV offers increased capabilities. So almost all usage of relative timelocks has been with the CSV opcode, not with the raw `nSequence` value on its own.
| | Absolute Timelock | Relative Timelock |
|------------------|-------------------|-------------------|
| Lock Transaction | nTimeLock | nSequence |
| Lock Output | OP_CHECKLOCKTIMEVERIFY| OP_CHECKSEQUENCEVERIFY |
## Understand the CSV Opcode