A contentious area of business writing, knocking copy is defined as ‘advertising copy that attacks a rival’s product’.

Some advertisers suggest that knocking copy is a necessary way of getting a forceful point into potential customer’s minds: ‘JunkAir is cheaper than TrashFlights’, for example. And it’s fair to say that knocking copy can be used to hilarious effect, such as the time Virgin Atlantic exploited the delay in erecting the London Eye to mock the wheel’s sponsors with the line: ‘BA can’t get it up’.

‘Comparative advertising’ — to give knocking copy its formal name — is a legal minefield and subject to strict rules stemming from an EU directive, so business writers should think carefully before ‘dissing’ the competition and its products.

The Plain Text school of thought suggests, however, that knocking copy is almost always a bad idea. Apart from legal difficulties, there’s the bare fact that in drawing attention to competitors, you are doing their promotional work for them. If the product can’t be sold on its own merits, don’t sell it.