Some might wonder, "Since any major and its equivalent minor have the same key signature, how do we know when we look at a piece of sheet music whether it is major or minor?"

Well, often we can't just by looking at it. We need to hear the tonal center. Major scales generally lend themselves to happy, grand or sweet melodies. Minor scales generally sound sad, melancholy, haunting. But these descriptions can be deceiving because some songs are not overtly happy or sad sounding. I didn't realize that "Walk On By" by Dionne Warwick was in a minor scale until I absently picked out the scale on a bass guitar just to amuse myself and realized that it was a minor scale.

That's why a lot of classical pieces were titled like "Etude in D minor" and like that so that the scale mode was apparent.