analog / analogue
The American and British spellings, and the contrast with digital.
Use analog for the adjective opposite digital: an analog clock, analog recording. Use analogue for the noun meaning a counterpart: a modern analogue of the old tax. The British spelling analogue covers both roles. Do not switch spellings inside one piece.
Examples
UseThey kept an analog line as a backup.
UseYeast is a useful analogue for human cells in that test.
AvoidThey kept an analogue line as a backup. (Not wrong in British copy; not the default here.)