A device is a thing: a tool, gadget or scheme. To devise is to invent or plan one. The pattern is reliable in British English — C for the noun, S for the verb — and it is the same as advice/advise and licence/license.
The core difference
A noun and a verb distinguished by one letter, following a real English pattern.
- device (noun, "-ss" sound) — a tool or scheme: a clever device.
- devise (verb, "-z" sound) — to invent or plan: devise a strategy.
The pattern
Advice/advise, licence/license, practice/practise, prophecy/prophesy. In British English the C spelling is the noun and the S the verb throughout. American English simplifies some of these — license and practice for both parts of speech — but device/devise is the same on both sides.
The sound helps
Unlike the others, this pair sounds different: the noun ends in an S sound, the verb in a Z. If you can hear which you are saying, you know which to write.
Frequently asked questions
Which spelling is the verb?
Devise, with an S. Device with a C is the noun.
What other words follow this pattern?
Advice and advise, licence and license, practice and practise, prophecy and prophesy.
Does American English differ?
For some of the pattern, yes — license and practice serve both roles. But device and devise are the same in both varieties.