Martin Underwood:
It was my first thought when I heard people referring to the
Twin Towers attacks as "9/11".

Stephen Farrow:
Though, to be fair, the emergency services number is always read as
"nine-one-one", never as "nine-eleven".

Colin Rosenstiel:
So my US citizen brother confirms today. So why the 9/11 epithet for the
bombing of the twin towers? ...

"9/11" properly refers to the entire attack, not just that part of it:
the hijacking of four planes, the destruction of the World Trade Center,
the plane crashed into the Pentagon, and the plan to similarly attack
another Washington site (probably the Capitol), which was thwarted.

People, especially politicians, needed a way to refer inclusively to
all parts of the attack. The attack on the WTC was clearly the most
desctructive part, but anyone in public life outside of New York who
referred only to that, the way Martin and Colin did above, would risk
being seen as slighting the other victims. (Suppose people came to
refer to this month's attacks as "the Tube bombs" -- wouldn't that
make you cringe if someone close to you was on that #30 bus?)

But it was hard to find a terse expression that was also as inclusive
as desired. What the politicians quickly settled on was the phrase
"the events of September 11"; but that's still rather unwieldy. Since
the resemblance of the date 9/11 to the phone number 911 had already
been widely noted, and yet their pronunciation was different, the
shortening to "9/11" was a very natural thing.
--
Mark Brader | "Follow my posts and choose the opposite
| of what I use. That generally works here."
Toronto | --Tony Cooper

My text in this article is in the public domain.