Increasing fourths and decreasing fifths are the terms used to describe intervals in theory and are an integral part of performance. An augmented fourth is a measure of a fourth on an interval, but one semitone greater than a perfect fourth. It can also be referred to as a reduced fifth, because it is actually a reduced fifth relative to the interval of the basic scale. In the case of the C note, for example, the interval between the C note and the F note is an increase of a fourth. In **, this interval is often used to create a tense, unstable, or special color effect. 
Similarly, diminished fifth refers to a fifth in an interval but one semitone less than a perfect fifth. It can also be called an increased fourth, because it is actually an increased fourth interval relative to the interval of the basic scale. For example, the interval between the C note and the GB note is minus fifths. This interval is also often used to create special effects in **. In general, the increase of the fourth and the decrease of the fifth degree, as important concepts in the theory, play an important role in the creation and performance. Their use can add a wealth of color and emotion, making it more vivid and interesting. 
In addition to playing an important role in the theory, the increase of the fourth and the decrease of the fifth also have a wide range of applications in the creation and expression of the first. In composition, these intervals are often used to construct chords, melodies, and sound effects to create unique styles and effects. For example, in pop**, increasing fourths and decreasing fifths are often used to construct chord progressions to create special harmonic effects. In genres such as rock and metal, the increase of fourths and decreases of fifths are widely used to create intense sound effects and unique melodies. 
In addition, in playing, the increase of the fourth and the decrease of the fifth are also often used to create unique sound effects and techniques. For example, in guitar playing, you can use an increase of four and a decrease of five degrees to create unique portamento, tremolo, and strumming effects. In short, increasing the fourth and subtracting the fifth are not only important concepts in the theory, but also have a wide range of applications in the creation and performance. Their use can add a wealth of color and emotion, making it more vivid and interesting.