A black cab is a type of hackney carriage (taxicab, An Austin FX4 or a Metrocab) common in Britain.
The sessions are recorded while the black cab that serves as the studio travels through city streets, usually in London, England.
For example, during the New Pornographers session, the two musicians can be seen bouncing up and down in their seats as the cab hits speed bumps.
The performances are also shaped by the limited space afforded by the back of a black cab.
In at least one instance, when singer/harpist Joanna Newsom was invited to perform, the fact that her harp would not fit in the cab kept her from contributing at all.