Heart, head, gut... and soul (missing from the theory -- which is rare since the enneagram is a very spiritual-themed thing or at least it was conceived as such). Also, each fix has a separate instinctual stacking. An example of a quatrifix somebody could have is :

8w9 sp/so - 2w3 so/sx - 6w7 sp/sx - 9w1 sx/so


The soul fix can be any enneagram type independently. (I mean; it can be from any center, but it cannot repeat an already existing fix).

And each fix has a Socionics type.

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This is your lesson today kiddos