It doesn't even make sense now, they're not used in spectroscopy anymore, but this is where the names originally came from and they did stick.

现在看它没什么道理，它们在光谱学里也不这么用了，但这些名字,从这里面起源后来就一直沿用下来了。

麻省理工公开课 - 化学原理课程节选