Bass TONE of chords

The bass tone of a chord is the note name of the note that belongs to the chord and has the lowest pitch. For instance, the C major triad chord in the next figure has E4 as the note with the lowest pitch. Thus, E is the bass tone of this chord.  

Figure: The C major triad chord whose bass tone is E (the bass tone is marked in red)

The bass tone of a chord is important because it is used for classifying positions of chords. Root position is a position of a chord in which the bass tone is the same as the chord root. The next figure shows C major triad chords that are all in the root position. They all have their bass tones equal to the chord root C. The first one is in the root close position.

Figure: C major triad chords in root position (the bass tones are marked in red)

When we want to include the bass tone in a chord annotation, we do that by adding the slash (/) and the bass tone at the end of the chord annotation. For instance, the C major triad chord in the next figure can be annotated C/E.

Figure: The C major triad chord with the annotation that includes the bas tone

When the chord is in a root position, we do not annotate the bass tone. Often, bass tones are not annotated at all. In such a case, the absence of bass tones does not mean that all the chords are in the root position. However, when bass tones are annotated, a chord that is written without a bass tone is in the root position.