A correspondent writes to say that he has had his writing criticised by his boss for 'faulty parallelism' and he wonders what it is.
It's a somewhat vague term, I have to say, but I imagine what the boss is concerned about is the kind of thing which Fowler called 'parallel-sentence dangers', and which has thus attracted especial attention from stylists over the years. Any piece of discourse can be said to show structural parallelism (as it's usually called) if it uses the same construction more than once in succession. There's a discussion of it in The Comprehensive Grammar of the English Language (section 19.7) and also at various places in my Making Sense of Grammar (see the index).
Parallelism is most obvious at sentence level (In the north it will be sunny. In the south it will be wet.). Here are two famous examples:
I came. I saw. I conquered.
One Ring to rule them all. One Ring to find them.
But it is also to be found within the sentence (e.g. Like father, like son), where often the parallelism is signalled by particular words (e.g. not only ... but also) or by a distinctive string (I left my new and fashionable coat in an old and unfashionable cafe).
Parallelism goes wrong when someone attempts to introduce a parallel construction, but doesn't quite make it. Here are some examples:
I want a new car and to sell my old one.
The paper described both the London marathon and New York.
We like to swim in the sea and surfing the waves.
The critics have been more critical, nasty, and angrier than I have noticed before.
The weather in Spain is drier than Portugal.
The answer is either right or an error.
Notice that most cases involve comparisons or lists. The easiest solution, accordingly, is to check that each element has the same structure.
a new car
to sell my old one.
better rewritten as:
to buy a new car
to sell my old one.
Developing an ear which is sensitive to unbalanced structures is part of the process of acquiring a mature style. I always find that reading a sentence aloud can help, as genuine structural parallelism is usually reinforced by a balanced rhythmical structure.