Which tty gets the X server?

[MR ZenWiz]

Having just crashed my X server a few times (Libre/Open Office issues
- not here), I noticed something interesting.

I thought the X server (gdm, etc.) ran on tty7, with ttys 1-6 being
the alternate, text-only virtual consoles.  After crashing, it seems
that the X server moves to tty8 and even tty9 (if tty8 crashes).

Can anyone verify (and explain) this?  Seems pretty strange to me....