In music theory, two of the building blocks of musical form are phrases and periods. A musical phrase is the smallest unit of music with a defined beginning and end. Most musical phrases consist of a beginning I chord progressing to a IV or a V chord and ending again on the I chord. Theoretically, thousands of chord progressions may exist between that first I chord and the IV or V chord. However, you may lose your audience in that time.
Musical phrases are like sentences in a paragraph — just as most readers don’t want to wade through a thousand lines of text to find out the point of a sentence, most music audiences are listening for the musical idea expressed in a phrase and get bored if it sounds like you’re just meandering between chords and not coming to a resolution.
So how long should a musical phrase be? It’s really up to the composer, but generally, a phrase is usually two to four measures long. Within that space, a phrase begins, works through one or more chord progressions, and resolves itself back to the I chord.
When a composer really wants you to understand that a group of measures are to be linked together in a phrase and played as an important unit — kind of like a topic sentence in an essay — he or she links the phrase together with a curved line called a phrase line. Notice how the phrase both begins and ends on the I chord, or the G major chord. (The numbers above the Roman numerals are fingering markings for the pianist; you don’t need to worry about them.)
Don’t confuse phrase lines with ties and slurs. A phrase line ties an entire musical phrase together, whereas slurs and ties only tie together a small part of a phrase.
The phrase represents the smallest unit ending with a cadence in a piece of music. The next larger unit used in musical form is the period. Musical periods are created when two or three musical phrases are linked together. In the case of musical periods, the first musical phrase is one that ends in a half‐cadence (ending at the V/v chord), and the second phrase ends with an authentic cadence (ending with the V/v chord resolving to the I/i chord).
The half‐cadence comes across like a comma in a sentence, with the authentic cadence, or consequent, ending the linked phrases like a period.
Here’s an example of a musical period.
