Maximum of 9 file descriptors

[JD Ballard <i.am.qix@xxxxxxxxx>]

Is there a reason why file descriptors are maxed out at 9?

    $ zsh
    $ cat 9>/tmp/foo # works
    $ cat 10>/tmp/foo # error

With Bash, this isn't the case.

    $ bash
    $ cat 9>/tmp/foo # works
    $ cat 10>/tmp/foo # also works