To say more, since people are asking: equal temperment means that you take an octave and divide it into 12 pitches, so that the ratio of each pitch to the next

To say more, since people are asking: equal temperment means that you take an octave and divide it into 12 pitches, so that the ratio of each pitch to the next is the same (the 12th root of 2). When you play in a major key, you pick a particular set of 7 of those 12, with a particular set of "skips". You can get from one major scale to another one by multiplying all the frequencies by the same scaling (some power of the 12th root of 2). Notice that in that major scale, for instance, the frequency between notes 1 and 7 is close to, but not exactly 3/2. It's in some sense out of tune. Back in the day it was more common for pianos to not be in equal temperment. One advantage might be that you could, in some key, have notes 1 and 7 be closer to a ratio of 3/2. But this means that the same scale in different keys would have different ratios of frequencies! That's how they would sound different.