is there a single word to represent "once every nine months"? Since 9 months is a fraction larger than half a year, semi-annualy seems wrong.

  • 1
    You could always try tri-trimesterally. What's the real-world context where you might need such a term? Jul 15 '14 at 19:09
  • Semi-annual (or biannual) is certainly wrong. I think you need to coin something. I like FumbleFingers' tri-trimesteral, but would add two more possibilities, nonamesteral and sesquibiannual.
    – Rupe
    Jul 15 '14 at 19:14
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    If you want something that people will actually understand, you can stick with "9-month period". Anything else could be confusing, IMO. (FumbleFingers Tri-Tri... sounds a lot like stuttering to me! lol!) Jul 15 '14 at 19:22
  • 1
    @KristinaLopez That is a fair point, although for the frequency the OP is looking for they'd still have to put something like their "once every" before it.
    – Rupe
    Jul 15 '14 at 19:29
  • 1
    every three quarters in economics; quarterly; bi-annually, every three quarters or every third quarter;
    – Lambie
    Feb 4 '17 at 17:38

Perhaps consider dodranial or perhaps dodrantal, formed from the Latin dodrans. Of dodrans, Wikipedia says:

The Latin word literally means nine-twelfths, and can refer to a time span of forty-five minutes or a length of nine inches.

Note, ignore the claim at languagehat/dodrans that dodrans means 2/3; a comment later in the page substantiates that dodrans is 3/4, while bes is 2/3. Also, phrontistery has an OED-based entry for dodrantal:

adj 1656-1883 of nine inches in length

  • I'd coin a word: Dodrennial. But hell yeah. Nov 17 '16 at 23:54

trimester = trimestris = tres + mensis = 3 months

semester = semestris = sex + mensis = 6 months

So wouldn't it be nomester = nomestris = novem + mensis = 9 months

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