If the vowels in the text are predominantly EH, IH, or EE, this may contribute to flatting since many choruses and quartets control these vowels poorly. Singers sometimes have only one area of two or three tones that are sung off-pitch habitually. This is most frequent on the several notes just below the break. Melodies that descend and suddenly turn upward are harder to keep in tune. Intervals that are repeated in sequence tend to lose their proper interval relationship in a short time.
Many diphthong combinations can cause flatting.