Argthtjtr

Is there a prefix or adjective that means "one and a half", as "tri-" or "triple" is for "three"?

The exact usage I have is to describe "18" in terms of a dozen. Where I live they've started making 18-egg cartons, and the local grocer was wondering what name to give them in his computerised cash register, hoping for a single word.

Even slang, informal or invented term would do if there isn't such a term already.

Sesqui-.

However, it certainly isn't informal, and is also pretty obscure. About the only use of it is sesquicentennial and sesquipedalian which is at a few removes (it literally means 18inches long but it's originally figurative meaning of using long words is the only English meaning).

It certainly wouldn't go with dozen; one would just say "one and a half dozen", "dozen and a half" or "eighteen".

Edit:

If you really want a word meaning 18 of something, you've got octodecuple, which has been used in mathematics at least as far back as 1816 and probably earlier. But really, don't; unless you're talking about properties of mathematical tuples where octodecuples are interesting in some particular way, it would just be horrible.

One does occasionally hear the term three-halves.

An on-line search for three-halves yields many results, including

• this image ³
  
  


• many references to Games of Three-halves, including Three-sided football ⁴, three types of 'football' played in Northern Ireland ⁵
  


• a theatre performance ⁶
  


• three-halves as a Superparticular number ⁷
  


• and numerous other varied references.

The expression three-ha'-pence was in common usage in the UK until 1971 as a common way of expressing 1½d (one-and-a-half pence) in the pre-decimal currency system (1½d = 1/160th of a pound sterling £).

An English three-halfpence silver coin was minted around 1561-1582 during the reign of Elizabeth I. ⁸

British three-halfpence silver coins were produced for use in certain British colonies at various times during the reign of Queen Victoria (1819 - 1901)⁹, as illustrated ¹¹:

_{P.S. In answer to the original question, I would not suggest using the expression three half-dozens unless you want to test your readers / listeners!}

The prefix that means "times one and a half" is sesqui-, as in sesquicentennial for the 150^th anniversary of something.

The OED includes the following terms that begin with sesqui-:

sesquialter, sesquialtera, sesquialteral, sesquialterate, sesquialterous, sesquibasic,
sesquicentenary, sesquicentennial, sesquicompound, sesquiduple,
sesquiduplicate, sesquinonal, sesquiocellus, sesquioctaval, sesquioxidation,
sesquioxide, sesquioxidic, sesquioxidized, sesquipedal, sesquipedalian,
sesquipedalianism, sesquipedality, sesquiplane, sesquiplicate, sesquiquadrate,
sesquiquarta, sesquiquartal, sesquiquintile, sesquiseptimal, sesqui-square,
sesquiterpene, sesquitertia, sesquitertial, sesquitertian, sesquitertious.

So it is hardly informal or slang; anything but, actually: it's rather a bit fancy.

Another, rather archaic ordinal description is "half again as." That has the connotation of "half more" over one.

Hendecasyllabic is derived from Greek for an 11 syllable word litterally meaning 11 sounds. The greek prefix for 18 would be octodeca- I wouldn't know how to form a noun from it. But here is a great resource on greek and latin numerical prefixes