Defined in my 2018 paper, externalisability, together with its dual property of addressibility, represents an essential property of a substrate supporting open authorship. An externalisable design is one which is serialisable in a form in which every design element has more or less stable and legible coordinates. An addressible, externalised serialisation of a design could be in terms of a JSON document with human-readable keys, to be contrasted with relatively opaque kinds of serialisation such as Python’s pickling or Java’s object serialization systems, designed to round-trip arbitrary in-memory structures allocated in a general-purpose programming language.