Runes

Runes allow Bitcoin transactions to etch, mint, and transfer Bitcoin-native digital commodities.

Whereas every inscription is unique, every unit of a rune is the same. They are interchangeable tokens, fit for a variety of purposes.

Runestones

Rune protocol messages, called runestones, are stored in Bitcoin transaction outputs.

A runestone output's script pubkey begins with an OP_RETURN, followed by OP_13, followed by zero or more data pushes. These data pushes are concatenated and decoded into a sequence of 128-bit integers, and finally parsed into a runestone.

A transaction may have at most one runestone.

A runestone may etch a new rune, mint an existing rune, and transfer runes from a transaction's inputs to its outputs.

A transaction output may hold balances of any number of runes.

Runes are identified by IDs, which consist of the block in which a rune was etched and the index of the etching transaction within that block, represented in text as BLOCK:TX. For example, the ID of the rune minted in the 20th transaction of the 500th block is 500:20.

Etching

Runes come into existence by being etched. Etching creates a rune and sets its properties. Once set, these properties are immutable, even to its etcher.

Name

Names consist of the letters A through Z and are between one and twenty-eight characters long. For example UNCOMMONGOODS is a rune name.

Names may contain spacers, represented as bullets, to aid readability. UNCOMMONGOODS might be etched as UNCOMMON•GOODS.

The uniqueness of a name does not depend on spacers. Thus, a rune may not be etched with the same sequence of letters as an existing rune, even if it has different spacers.

Divisibility

A rune's divisibility is how finely it may be divided into its atomic units. Divisibility is expressed as the number of digits permissible after the decimal point in an amount of runes. A rune with divisibility 0 may not be divided. A unit of a rune with divisibility 1 may be divided into ten sub-units, a rune with divisibility 2 may be divided into a hundred, and so on.

Symbol

A rune's currency symbol is a single Unicode code point, for example $, ⧉, or 🧿, displayed after quantities of that rune.

101 atomic units of a rune with divisibility 2 and symbol 🧿 would be rendered as 1.01 🧿.

If a rune does not have a symbol, the generic currency sign ¤, also called a scarab, should be used.

Premine

The etcher of a rune may optionally allocate to themselves units of the rune being etched. This allocation is called a premine.

Terms

A rune may have an open mint, allowing anyone to create and allocate units of that rune for themselves. An open mint is subject to terms, which are set upon etching.

A mint is open while all terms of the mint are satisfied, and closed when any of them are not. For example, a mint may be limited to a starting height, an ending height, and a cap, and will be open between the starting height and ending height, or until the cap is reached, whichever comes first.

Cap

The number of times a rune may be minted is its cap. A mint is closed once the cap is reached.

Amount

Each mint transaction creates a fixed amount of new units of a rune.

Start Height

A mint is open starting in the block with the given start height.

End Height

A rune may not be minted in or after the block with the given end height.

Start Offset

A mint is open starting in the block whose height is equal to the start offset plus the height of the block in which the rune was etched.

End Offset

A rune may not be minted in or after the block whose height is equal to the end offset plus the height of the block in which the rune was etched.

Minting

While a rune's mint is open, anyone may create a mint transaction that creates a fixed amount of new units of that rune, subject to the terms of the mint.

Transferring

When transaction inputs contain runes, or new runes are created by a premine or mint, those runes are transferred to that transaction's outputs. A transaction's runestone may change how input runes transfer to outputs.

Edicts

A runestone may contain any number of edicts. Edicts consist of a rune ID, an amount, and an output number. Edicts are processed in order, allocating unallocated runes to outputs.

Pointer

After all edicts are processed, remaining unallocated runes are transferred to the transaction's first non-OP_RETURN output. A runestone may optionally contain a pointer that specifies an alternative default output.

Burning

Runes may be burned by transferring them to an OP_RETURN output with an edict or pointer.

Cenotaphs

Runestones may be malformed for a number of reasons, including non-pushdata opcodes in the runestone OP_RETURN, invalid varints, or unrecognized runestone fields.

Malformed runestones are termed cenotaphs.

Runes input to a transaction with a cenotaph are burned. Runes etched in a transaction with a cenotaph are set as unmintable. Mints in a transaction with a cenotaph count towards the mint cap, but the minted runes are burned.

Cenotaphs are an upgrade mechanism, allowing runestones to be given new semantics that change how runes are created and transferred, while not misleading unupgraded clients as to the location of those runes, as unupgraded clients will see those runes as having been burned.