Equal Temperament gradually took over from the various 'well-temperaments' at the end of the 19th century and has been the 'standard' tuning system for keyboard instruments ever since. Each semi-tone is exactly 100 cents wide, producing a tuning system which is identical in all keys, allowing free modulation to any key.

However, the history of equal temperament goes back much further than this. It was accepted as the standard tuning for the lute by the mid 1500s, as a compromise solution, but really the only way to tune a fretted string instrument. There is a written reference by Abbott Girolamo Roselli in 1588 and therefore it is appropriate to ask why it was not adopted for keyboard instruments until much later.

Part of the answer is that it is more difficult to tune keyboard instruments to Equal Temperament than to Pythagorean Tuning or Meantone tunings, but a more convincing answer can be seen if Equal Temperament is viewed relative to Natural Tuning. The 5ths and 4ths are good, but both ,major and minor 3rds are significantly different to the natural ratios, which would have been unacceptable in Baroque and Classical periods. A rather more contentious difference can be seen by comparing Equal Temperament with a well-temperament. Looking at the tunings relative to themselves, Equal Temperament is identical in all keys but Werckmeister has different intervals in different keys, 'colouring' the keys. This move towards homogeneity has not always been viewed as an advantage! (Dumbing down 19th century style?)

One last thought. It is a strange coincidence that after a journey through a plethora of meantone tunings and well-temperaments, we have arrived in the 21st century at a tuning, in Equal Temperament, which is relatively close to the Pythagorean Tuning of Medieval times. Looked at relative to each other, the 5ths are extremely close and the 3rds are both sharper than their natural ratio and relatively close at only 8 cents apart, so modern Equal Temperament is better choice for playing Medieval music than all of the tunings and temperaments in between.