The material explained below is based on the original Derbyshire theme. Subsequent versions of the theme will be covered in the future, and the ways in which they differ will be covered in their individual sections.
The Doctor Who theme's bass part can be thought of as a series of discrete "blocks." An individual block of the bassline is a short (½-bar) section. There are three basic types of block, which are named onomatopoeically: the dum-de-dum, the diddly-dum, and the dum-dum-diddy.
A dum-de-dum is a ½-bar-long section of the bassline where the rhythm consists of a note two triplets long, a note one triplet long, and a note one quarter note long, as shown above.
A diddly-dum is a ½-bar-long section of the bassline where the rhythm is three notes, each one triplet in length, followed by a note one quarter note long.
A dum-dum-diddy is a ½-bar-long section of the bassline where the rhythm is two notes two triplets in length, followed by two notes one triplet in length.
There are two types of dum-dum-diddy:
- High-low dum-dum-diddy (in the form G, G, F, D or D, D, C, A)
- Low-high dum-dum-diddy (in the form B, B, C, D)
Note that dum-dum-diddies only go in one direction, meaning either descending ("high-to-low") or ascending ("low-to-high"). They do not change direction partway through (such as D, D, C, D).
In addition to the main bass layer (the first layer) outlined above, there is an important and oft-overlooked second layer to the bassline: the grace notes.
The bassline's second layer crucially serves as a lead-in to each bassline block. The second layer begins on the note that is one note lower in the scale than the first note of each block. (Usually this is two semitones down, but in a few instances it is only one.) That note then leads directly into the first note of each block. Some themes glide between these notes, while others do not.
Provided below are three clips demonstrating the component parts of the bassline. The first clip demonstrates the first layer on its own, the second demonstrates the second layer on its own, and the final clip illustrates the two bassline layers combined (as they are normally heard in the theme).