Halliday & Matthiessen (2014: 167):
The fundamental opposition is that between falling and rising; the whole of the tone system can in fact be constructed out of that simple contrast. At the most general level, falling tone means certainty, rising tone means uncertainty. A neutral, more or less level tone, is one that opts out of the choice. There are then two possibilities for forming more complex tones: falling-rising, which means something like ‘seems to be certain but isn’t’, and rising-falling, complementary to that, which means ‘seems not to be certain but is’. This defines the five simple tones of spoken English. In addition, two compound tones are formed by adding the neutral tone to one that ends with a fall. The simple tones are numbered 1 to 5, the compound ones 13 and 53 (‘one three’, ‘five three’).