Free A Player's Guide to Chords and Harmony by Jim Aikin Page B

diatonic triads in a given key are referred to as the I, II, III, IV, V, VI, and VII chords, as shown in Figure 4-5.
    Some musicians prefer to use lower-case Roman numerals to indicate the minor diatonic triads (ii, iii, and vi). This system is somewhat useful, in that it allows you to indicate quickly the difference between a ii chord (which is minor) and a II (which is major, although it's not a diatonic triad because the 3rd is an altered scale step). However, the upper/lowercase system is not universally employed. If you see the notation II or III, you can't be certain that whoever wrote it meant you to play a major chord.
    Occasionally you'll see this numbering system extended to refer to chords whose roots are not in the diatonic scale. A 6VI chord, for instance, is one whose root is on the lowered 6th degree of the scale (A6 in the key of C), and so on. This system has an inherent weakness when it comes to verbal communication, however. Some of the chords we'll discuss later in this book include a flatted 5th, and are called "flat-five" (65) chords. A 65 chord is not the same thing as a 6V chord. One is a triad with a lowered 5th, and the other is a chord built on the flatted 5th step of the scale. But in most situations, it will be clear from the context which type of chord the speaker is referring to.
    The practice of referring to chords by number is very commonly used. Improvising musicians talk routinely about various sorts of II-V progressions, for instance. If a musician says to you, "That's a two-five in the key of B6," he or she is talking about a progression in which some variety of C chord (the II in the key of B6) is followed by some variety of F chord (the V in B6).
    We humans have a penchant for giving names to things, so maybe it's not surprising that there's another whole set of terminology that can be used to refer to the same group of diatonic chords. As Figure 4-6 shows, each note (scale step or chord root) in the major scale has a name. The I note/chord we've already seen referred to as the tonic, but the other terms are new.

    Figure 4-5. The diatonic triads in a given key are often referred to using Roman numerals. Purely for the sake of variety, this example is given in the key of E.

    Figure 4-6. The scale steps also have names, as shown here for the key of C major.
    The most important terms in Figure 4-6 are "dominant" and "subdominant," for reasons discussed in the next section. The term "leading tone" is also important, but for slightly different reasons. The terms "supertonic," "mediant," and "submediant" are rarely heard outside of music theory classrooms.
    The term "leading tone" almost always refers to the single note below the tonic, not to a chord built on that note. This note is called the leading tone because in a V-I progression, the 3rd of the V chord almost always moves upward to the root of the I chord (see Figure 4-7). In the natural minor scale (see Chapter Seven), the note below the tonic is sometimes referred to as the subtonic, because it doesn't tend to move upward to the tonic.
    Other than in classical music, chords are seldom built using the leading tone as a root, because the diminished triad is felt to be too unstable. Rock songs often use the major triad built on the lowered VII, however. In classical music, the VII triad sometimes substitutes for a V7 chord (see Chapter Five), because it contains all of the notes of the V7 except the root. In this usage the VII chord is almost always in first or second inversion.

    Figure 4-7. The leading tone (B in the key of C major) gets its name from the fact that in a V-I progression, the leading tone almost always moves upward to the tonic.
     

FIGURED BASS
    Musicians in the Baroque period had a system of symbols that could be used for writing out chord charts. Just as jazz players do today, harpsichordists and other instrumentalists would improvise their own parts based on the symbols in the chart. This system, which is called