Why Forty, not Fourty?

Date: 3/30/96 at 16:18:19
From: Anonymous
Subject: Spelling of forty

My 7th and eighth grade math classes from Rossville, Kansas have 
tried to figure out why the number "forty" is spelled as it is and not 
"fourty".  Dictionaries and encylopedias haven't been any help.

Would it have any relation to a Fort where there may have been 40 
involved?

Date: 3/30/96 at 17:22:8
From: Doctor Sarah
Subject: Re: Spelling of forty

Hello there -

Trying to figure out why something is spelled the way it is in 
English is often frustrating.  :-)  Spelling didn't become very 
'stable' until well after the time of Shakespeare; if your 7th 
and 8th graders haven't seen Shakespearean spelling yet, it might 
be an eye-opener for them.

A better question to ask of a good etymological dictionary like 
Webster's Second International might be where the words four and 
forty come from.  My dictionary says this: 

ME. is Middle English, the language of England between about 
1100 and 1500 A.D.

AS. is Anglo-Saxon, the language of the Saxon tribes that invaded 
England in the 5th and 6th centuries - from about 600 A.D.

OS. is Old Saxon, the language of the original Saxon tribes of 
northwest Germany between the Rhine and the Elbe rivers.

forty:  ME. forti, fourti, fowerti, from AS. feowertig;
        akin to OS. fiwartig, fiartig

four:   ME. four, fower, feower, from AS. feower;
        akin to OS. fiwar

So four and forty were different words starting a long time ago but 
were spelled with the same beginning in Old Saxon and Anglo-Saxon 
and part of Middle English.  Somewhere along the way during the 
Middle English era the simpler spelling of forty took hold and has continued ever since. 

Of course it was easier to change spellings before there were 
dictionaries and teachers paying close attention to your spelling. 
:-)

It's interesting to notice the British spellings of words like 
humour and labour, etc., which are spelled humor and labor in 
the United States.  

I hope this helps answer your students' question.

-Doctor Sarah,  The Math Forum