Thematic design and phrase structure
The 'theme' in music usually refers to the principal melodic lines. The tonal motion towards cadences is supported by the presentation and completion of thematic ideas. A phrase is usually notated with a slur:
Fig.1 Slur
Part II - Motives and themes
A musical motive is a short melodic idea categorized by rhythm, contour and interval succession. a theme is a sequence of motives that make a cohesive whole. Motivic analysis usually entails the identification of motives and their recurrences.
Part III - Models of Phrase and structure
A common model of phrase structure is a sentence. A sentence is a phrase comprised of a statement, followed by a repetition, then a continuation that leads to a cadence. The three parts of a sentence are normally in the proportions 1:1:2 (in measures) or multiples thereof. The proportions are the defining characteristic of a sentence.
Another type of phrase is the period. The first phrase in a period leads to a weak cadence (IAC or HC). This first part of the period is known as the antecedent. The second part of the period, known as the consequent, leads to a strong cadence (usually a, AC of some sort.
