That's one way of referring to it; see http://www.tafford.com/detail.htm?sid=NXPOBL

But there are many variations, for instance "placket front opening"; see http://en.wikipedia.org/wiki/Shirt#Types_of_shirt

It might also be called "x-button placket opening" (where "x" is "2" or "3" or "4', etc.).

See "References" below for Google Image search of "patte de boutonage" and dictionary reference for "placket."

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Note added at 26 mins (2011-07-19 14:40:33 GMT)
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Apologies to Roberta... For some reason her Answer was not visible when I was writing mine.

I have come across this term once before, but can't quite remember how I translated it.
generally, 'patte' = 'tab' in many instances.

Usually, of course, this tab actually carries the buttonhole — however, it is a 'tab used for buttoning', and I think 'button tab' is OK; you could certainly use it as a starting point for further research.

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Note added at 1 heure (2011-07-19 15:19:11 GMT)
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This little sewing lexicon explains a bit:

http://www.petitcitron.com/index.php?option=com_glossary&tas...

Here's another pags from the same site, with a picture:

http://www.petitcitron.com/index.php/fente-manche-patte-bout...

However, see this picture for a rather different type of 'patte', the one that I was describing above:

http://www.originalemode.com/produit--0/pantacourt-a-carreau...