Altered chords are chords that have been modified from their original form by changing one or more of their notes. This often involves raising or lowering specific notes, such as the fifth or ninth, to create a different sound. They are commonly used in jazz and contemporary music to add tension and color to harmonic progressions. These chords can include alterations like flat ninths, sharp ninths, or flat fifths, which provide a unique character. Musicians often use altered chords to create a sense of movement or resolution in a piece. They can be found in various styles, enhancing the emotional depth of the music.