A recorder quartet playing notes determined by the topography and geography of the route followed by the 31 bus. The piece, like the bus journey lasts 22 minutes.
Musical notes are assigned from the numerical data, such as map references and heights, by simple substitution, so, for example, 3 becomes G and 7 becomes A In the case of the street names, where a letter of the alphabet is not a note from the musical scale, it is substituted systematically for one that is. Thus CASTLE becomes CAEFEE and so on.

The tenor recorder plays notes derived from the name of the road the bus is on at the time. Each musical phrase lasts the same length of time as the bus spends on that road.

The treble plays a set of phrases dictated by the distance travelled so far, the compass bearing, the time elapsed and the change in direction since the last checkpoint, which are at 100m intervals. The phrase plays for the 24 seconds that the bus spends, on average, on each 100m section. Most of the time each phrase is played twice, but on the steep uphill sections it plays once, while on the steep downhill parts it plays three or even four times in 24 seconds.

The descant line represents the spot heights above sea level at 36 points along the route. The 8 note phrase is derived from the grid reference, while the speeded up repeat of the phrase is played once for each 10m above the level at the start of the journey.

The sopranino "chimes" every minute with a "pip" added for each minute elapsed.
The four parts were recorded separately and digitally edited to conform, to within a few milliseconds, to the appropriate parts of the bus route. Finally the tracks were mixed together into one recording. There has been no attempt to make the sounds into a pleasing musical composition, the rules are strictly adhered to - at any given moment the combination of notes heard is entirely dictated by the rules set at the outset.