A hemiola is a musical section that is technically two bars long but plays like three, or vice versa. The technique was popular during the Baroque era, and many Viennese waltzes employed it. In essence, musicians play two triplets in 3/4 time as three duplets in 2/4 time. This can be difficult to hear in a piece of music, but the accents used at the beginning of each bar help listeners notice the difference. Square brackets will typically surround groups of notes in a hemiola to indicate how they should be played.
To comprehend what a hemiola is, a basic understanding of musical time signatures is required. Music is divided into bars, which are defined as predetermined periods of time with a predetermined number of beats. The bottom number in the time signature represents the length of time, while the top number represents the number of beats. A bar in 4/4 time, also known as common time, has four beats evenly spaced at quarter-bar intervals. Only in 3/4 time, where three beats are spaced out over a bar at one-third bar intervals, can a hemiola occur.
There will be three quarter notes spread out over the length of a standard bar of 3/4 music. The analogy of a clock face can help laypeople grasp this concept. The beats in a standard 4/4 bar are at the 12 o’clock, three o’clock, six o’clock, and nine o’clock positions. The beats in a 3/4 bar are at 12 o’clock, four o’clock, and eight o’clock.
Because the length of a bar is not a fixed term, two bars of 3/4 time and three bars of 2/4 time both have six evenly spaced beats. This means they are difficult to distinguish from one another. Accenting, or playing the first beat of a bar with emphasis, is the only way to indicate the beginning of a bar. In a hemiola, every second beat is accented as if it were in 2/4 time, rather than every third beat as in 3/4 time.
Square brackets are commonly used in musical notation to indicate when musicians should perform a hemiola. If a square bracket is placed over a group of three notes, the first of the three notes should be accented, as if the three notes were all together. In a hemiola, square brackets divide the notes in two 3/4 bars into two groups of two. This causes the emphasized note to shift, which is a common effect used Baroque musicians.