A Thurberesque interval

The American author and humourist James Thurber delighted in language and word games. His (notionally) children’s book “The Wonderful O”, for example, is a joyous romp through the language.

One of my favourite Thurber asides takes place while he is in hospital. He quizzes a nurse “What’s a seven letter word containing three “u”s?”. She ponders for a moment, and replies “It must be unusual…”, to which Thurber responds “It is, and it isn’t”.

So, in memory of James Thurber, here’s a little puzzle. What is it that never stands alone, yet without it as the vital ingredient we can’t pro-, re-, in-, e-, di-, trans-, ag-, or retro-, and without which we’d lack o-, ti-, con- and ne-?

Give up?