What determines location of :0, :1 & :2?

[Felix Miata]

Once upon a time not too very long ago, one could in most distros count on :0 
being on tty7, :1 on tty8 & :2 on tty9. Since about the time systemd started 
usurping sysvinit, this has less often been the case. See e.g. 
https://bugzilla.novell.com/show_bug.cgi?id=768788 . Some distros think X 
belongs on tty1. (I believe boot messages should go there, and stay there.)

What is it that determines which ttys are used for X sessions, and which 
remain reserved for framebuffer console sessions? I see that for KDM, in 
kdmrc there are several related config lines that serve to keep its :0 on 
tty7, but what about when KDM isn't used, such as when KDM is already running 
on :0 on tty7, and another session is opened on :1 via startx? Why should 
anyone need guess which tty holds which session rather than expecting them in 
traditional locations? IOW, what does one need to do to ensure ttys 1-6 are 
not usurped by any X session, and that :0 goes to tty7, :1 to tty 8, and :2 
to tty9?
-- 
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  Team OS/2 ** Reg. Linux User #211409 ** a11y rocks!

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