pair

The pair was originally used for two things that always occur together, as in a clean pair of heels, a pair of rosy cheeks.  Then it was used for an object comprising two identical parts, neither of which existed on their own.  A pair of scissors, a pair of trousers. In addition, two people or things linked together in some way but not necessarily as a fixed set formed a pair.

 Some people argue that pair is a collective noun and should always take a singular verb. Others prefer to decide case by case. If the sense of plurality is uppermost then use the plural. If the singularity dominates then use the singular. 

 I favour the case-by-case approach, so I find it jarring sometimes when the singular is used where I would use the plural.  In Copspeak people are always ‘known to each other’.  This tells us that the event that took place was not random. There were connections and no doubt motives governing the participants.

 The use of ‘each other’ emphasises plurality so to say the pair was known to each other is very awkward.  The pair were known to each other seems to me to flow much more easily.  I don’t know if it is the police or the ABC who have made the style decision that pair must take a singular, but I wish they would be a bit more flexible.