Halliday & Matthiessen (2014: 116):
Each information unit is realised as a pitch contour, or tone, which may be falling, rising or mixed (falling-rising, rising-falling). This pitch contour extends over the whole tone group. Within the tone group, one foot (and in particular its first syllable) carries the main pitch movement: the main fall, or rise, or the change of direction. This feature is known as tonic prominence, and the element having this prominence is the tonic element (tonic foot, tonic syllable). We indicate tonic prominence by a form of graphic prominence: bold type for print, wavy underlining for manuscript and typescript. The element having this prominence is said to be carrying information focus.