Name for descending semitones under a repeating musical phrase?

Question

In much contemporary music, especially instrumental guitar music, there is a particular device employed. It involves a musical phrase in which usually one note, often a bass note, descends in one semitone intervals, while a(n often repeating) phrase is played.

Examples are in L'Amour Manque by Adrian Legg, the start of The Mystery and Footprints by Tommy Emmanuel, and the start of Stairway to Heaven by Led Zeppelin.

Given that this is so common, I wonder whether it's a recognised named musical device.

Answer 1

In popular music, this device is usually called Line Cliché. It is a chromatic line over a static chord creating the illusion of harmonic motion. Line chlichés are most often found in a minor key. The line usually moves near the 7th of the chord. A very common descending line (over a minor triad) is: root -> maj 7th -> min 7th -> maj 6th. This is exactly what you hear in Stairway to Heaven.

Answer 2

As Bob mentions, this can be described as a descending chromatic bass line. If descending a fourth, from tonic to dominant, this can also be called a "Lament Bass". As a technique, it dates back at least to the early Baroque Era (famously used in Dido's Lament, that character's dying aria from Henry Purcell's Dido and Aeneas, and also in Bach's Crucifixus from his Mass in B-minor). The figure is often used to convey extreme grief, sorrow, and death.

In general, a repeating bass line with other music played over it is called a Ground Bass, or Ostinato Bass. The lament bass (descending chromatic bass line) is commonly employed as a ground bass (as in Dido's Lament). A popular example of a ground bass that isn't a lament bass would be the bass line to Pachelbel's Canon in D, being a continuosly repeated pattern of eight notes.

Answer 3

A series of pitches which move down by semitones is a descending chromatic scale. If this line is in the bass you could call it a descending chromatic bassline.

If the harmony above this bassline remains unchanged, the resulting chords can be described as a series of inversions. In popular music terminology this is most easily achieved using "slash-chords". For instance, if a full C Major triad is played or held continuously over a descending chromatic bassline starting on the note C, the resulting chords could be described as:

C C/B C/Bb C/A C/Ab etc.

You could also take account of the interval added by the bass note:

C Cmaj7/B C7/Bb C6/A

This is particularly useful if you want to "allow" musicians playing the full chords to include the bass note in the chords/lines they play.

Also, it is worth noting that some of the resulting chords can be described differently. For instance, a C/A or C6/A chord can also be named Am7. The choice of chord name is probably best decided by context. If you want to make it clear that the harmony is static above a changing bassline, it would be best to keep the chord name the same (i.e. use C/x in the examples above).