Singers normally harmonize voices in homophonic parallelism moving in parallel thirds or fourths. Harmonization of the melody is accomplished by singing in parallel thirds, fourths, or fifths. Harmonies based on fourths and fifths also appear in folk guitar. In the 20th century, harmony explicitly built on fourths and fifths became important. Pieces may include quarters, or fourths of the chicken. Note the parallel fourths between the upper voices. Standard tuning mixes a major third (M3) with its perfect fourths. Ceaseless motion with lots of running fourths.