Traditionally, a tetrachord is a series of four tones filling in the interval of a perfect fourth. In modern usage a tetrachord is any four-note segment of a scale or tone row including the augmented fourth (+4). The term tetrachord derives from ancient Greek music theory, it literally means four strings.

A triad is a three note chord. In traditional chord theory there are four traditional triad chord types: major, minor, diminished and augmented. And four contemporary triad chord types: sus2, sus4, add2 and add9.

Intro to Improvisation using scales and and triads + 1 - lessons for developing the ability to improvise or "take a solo".