Equal temperament is a musical tuning system that divides an octave into twelve equal parts, known as semitones. This allows instruments to play in any key without sounding out of tune, making it easier for musicians to switch between different keys during performances. This system became widely adopted during the Renaissance and Baroque periods, as it facilitated the development of more complex music. Instruments like the piano and guitar are commonly tuned using equal temperament, enabling a consistent sound across various musical styles and genres.