The choice of ʻtoneʼ realizes the semantic values of KEY, which are related to the system of Mood. Strictly speaking ʻkeyʼ is outside the scope of the present volume, for precisely this reason: it is not expressed structurally, but prosodically, by the TONE CONTOUR, the melodic movement of the tone group.
- Halliday (1985), Pag. 281

In addition to its function in the expression of key, the system of melodic contours (the ʻtone systemʼ) also expresses certain logical relations between successive information units in a discourse. These two meanings are not as different as they might seem. Whereas ʻkeyʼ is the meaning of the tone in its paradigmatic environment, i.e., in association with other, non-tonal choices (those of mood), what we are describing now is the meaning of the tone in its syntagmatic environment, i.e., in succession with other tone choices.
- Halliday (1985), Pag. 285