How did equal temperament affect the sound of keyboard instruments?

Equal temperament fundamentally transformed the sound of keyboard instruments by enabling them to play in all major and minor keys with consistent intonation.1

Before its adoption, keyboard instruments relied on systems like meantone temperament or just intonation, which were based on acoustically pure intervals. These earlier systems worked well in simple keys but accumulated significant intonational errors when transposing to keys with many sharps or flats, often resulting in highly dissonant intervals known as "the wolf".12

By dividing the octave into 12 equal semitones, equal temperament distributes these small intonational defects evenly across the chromatic scale. While this means that no interval—except for the octave—remains perfectly pure, it allows for the expanded harmonic range required by 19th- and 20th-century music. For instruments like the piano and harpsichord, whose tones are already rich in dissonant components, this compromise is considered a practical and necessary advantage. However, some musicians have historically argued that the loss of purity is more noticeable on the organ due to its sustained, clearer tone.12