How to typeset the numero sign in Plain TeX?

Question

I want to typeset the numero sign № in Plain TeX.

My first guess is to use

\def\numero{$\rm N^{\underline o}$}

but the line under the o is too long and I don't really want to mess up with arbitrary length values to shorten the box.

And

\def\numero{$\rm N^{\b o}$}

does produce a normal-size o which is not what I want.

Any idea? I am fine with the o being superscript or baseline.

Answer 1

You can use some kerning.

\def\numero{%
  \leavevmode
  \hbox{%
    $\rm N^{\mkern0.8mu\underline{\mkern-0.8mu o\mkern-0.8mu}\mkern0.8mu}$%
  }%
}

\numero 1

\bye

Perhaps better, raise a smaller o with a bar under accent.

\def\numero{%
  {\rm N}%
  \raise1.05ex\hbox{\fiverm \b{o}}%
}

\numero 1

\bye

If you don't want to guess the amount of raising and use pdftex, you can use \fontcharht (or do it the hard way by measuring the height of N and doing some arithmetic).

\def\numero{%
  {\rm N}%
  \begingroup
  \setbox0=\hbox{\fiverm o}%
  \raise\dimexpr\fontcharht\font`N-\ht0\hbox{\fiverm \b{o}}%
  \endgroup
}

\numero 1

Answer 2

When you are using Unicode engine (xetex or luatex) then you can simply load a Unicode font and use № directly. For example:

\input lmfonts

№1

\bye

(process this example by luatex or xetex).