I've been playing and studying vibes for about 20 years. I've taken lots of classes and lessons ... but no one has been able to answer this question to my satisfaction --- why is the altered chord called the altered chord? In other words, what is altered in the scale of the altered chord that makes is so unique, it takes the name "altered chord"? Lots of scales and chords have alterations to some notes so that can't be the answer. Today I found a website that gave what I consider the perfect answer. Here it is:
[The altered chord] ... "comprises the three irreducibly essential tones that define a dominant seventh chord, which are root, major third, and minor seventh and that all other chord tones have been altered."
This is (in my humble opinion) the perfect answer. The root is there to tell you what chord it is, the third tells you if it's major or minor and the flat 7th tells you it's going to resolve to something. ALL the other scale tones are altered.
The website explains: "This scale has existed for a long time as the 7th mode of melodic minor." That makes it even easier to learn and play, just play over the melodic minor chord 1/2 step above the dominant 7th altered chord you're after. Well ... I've known this tip for years but never realized the root, major 3rd and flat 7th elements of this chord combined with all the other notes being altered. Makes perfect sense now.
Here's the website this came from: