Grey Knight FOC question

[Ensis Ferrae]

Ok, so since i have started into this whole world of 40k i have seen a number of army lists that have grey knights and regular marines in them at the same time... yet when i read in the Daemonhunters codex, it says in various places that that is not allowed. My question is, am I missing some sentence somewhere that does allow marines and GK in the same FOC?

[jester_prince]

The rules are a little strange when it comes to this. You CANNOT take a spacemarine attachment attached to a grey knights army or an army incluing them but you ARE alowed to attach grey knights to a spacemarine army.

[benmothershaw]

Yeah so A daemonhunters army with Grey Knights cannot include Space Marines as allies

 

Whereas a Space Marines army can use Grey Knights as allies.

 

Basically to represent Grey Knights not being allowed to employ other Space Marines from other chapters because they are a Space Marine chapter themselves not uber gods.

And the second to represent Space Marines calling for the aid of Grey Knights, or Grey Knights appearing to stop the taint of chaos reaching the space marine chapter. They would then of course Brain wash all the space marines that came into contact with Daemons.

[Grand Master Tyrak]

It also prevents the combination of Marine and GK Heavy Support.

The rationale behind this is that Space Marines are free to call on the Inquisition, but to call on the Space Marines, the Inquisition needs a damn good reason - like a major emergency that the Chamber Militant cannot get to in time. Inquisitors can spend years building up good relations with Chapters, they will not waste them on an emergency that the Chamber Militant can deal with. Plus a Chapter Master is well within his rights to refuse a request for aid, even if it does provoke a full investigation into the reasons. One does not refuse the Inquisition lightly, but the Space Marines and the AdMech are the only two organisations that have the (written) influence to do it.