Why is the Word "Number" Abbreviated to "No."?

Date: 05/13/2002 at 18:06:40
From: Angelica Rodriguez
Subject: Math: Why No.?

Dear Dr. Math,

Why is the word "number" abbreviated to "no."? 
                             
                         Sincerely,
                         Angelica

Date: 05/13/2002 at 22:49:23
From: Doctor Peterson
Subject: Re: Math: Why No.?

Hi, Angelica.

A lot of abbreviations in English are really abbreviations of 
words in other languages. In this case, "no." comes from 
"numero", which may be thought of as Italian, 

    http://www.dcita.gov.au/infoaccess/docs/chp7.pdf

    The word 'number' is frequently represented by no.
    (plural: nos) - the contraction of its equivalent in Italian
    (numero). 

    http://www.classics.cam.ac.uk/Faculty/grad/thesis3.html

    'no.' for Italian 'numero'

or as Latin:

    http://mathworld.wolfram.com/Number.html

    To indicate a particular numerical label, the abbreviation
    "no." is sometimes used (deriving from "numero," the ablative
    case of the Latin "numerus"), as is the less common "nr." 

The dictionaries I've seen agree with the latter, but I lean 
toward Italian. But it doesn't make much difference!

- Doctor Peterson, The Math Forum
  http://mathforum.org/dr.math/