A couple of weeks ago, the Guardian ran what they called 'a fiendish language quiz', set by their 'style guru', David Marsh, author of the recently published 'For Who the Bell Tolls: One Man's Quest for Grammatical Perfection'. In one of the questions, entrants had to say what was wrong with this sentence:
'Outstanding performances include Caryl Morgan's Beatie, in bobby socks and denim, on the cusp of womanhood but still impressionable and dreamy, and Sara Harris-Davies as the mother she still looks up to.'
The answers have now been published, and the official answer to this one was:
You can't be on the cusp of one thing. Although it is often misused to mean "on the brink" or "on the verge", strictly "on the cusp" involves two things, for example "on the cusp of Taurus and Gemini" or, as it should have been here, "on the cusp of girlhood and womanhood".
While taking the point that a cusp is between two things (often two periods), I wondered whether it was really necessary to explicitly mention both; on the cusp of womanhood looked fine to me. So I decided to investigate.