50|50 Words is a compilation of the dialects of reuse. It outlines a lexicon of language, is an expression of vocabularies, claims a glossary of terms all used to distinguish the transformation of the existing into something new. Its proposition is that an obsolete entity is not only a site of depredation, it is a condition for mediation and the location of research and design processes that will ensure that meaningful change through reuse will take place. In response to a world with finite resources and with the demise of extractivist approaches with which to remove them, the very-near future of the built environment will be focussed solely on the re-designation of all existing matter. New-build and single-use processes will be obsoleted distinctions for making cities, buildings, interiors and artefacts. 50|50 characterises centuries of these practices and it specifies their relevance to the 21st century, a time that can be characterised in its responses to the challenges of the climate emergency and social justices.