Halliday & Matthiessen (1999: 30):
Metalanguage has the same basic properties as any semiotic system. This means that it is stratified. It construes language in abstract theoretical terms; but this construal is in turn realised as some form of representation — either language itself, in discursive constructions of theory, or some form of designed semiotic (system networks, constituency rules, conceptual networks, logical formulae, and so on).