What are the pillars of Wing Chun ?

[T.D.O]

What are the main principles your Wing Chun system is based on?

Structure and energy....

[deejaye72]

on further thought; i'm thinking a pillar of the system would be facing. facing would reflect centerline theory. without the centerline we're not doing wing chun, is that not the first thing we learn?, we stand facing even before we get into yim yeung ma, when we learn siu nim tao. anyhow, good discussion!!

[BillC.]

Tan (Spread), Bang (Wing) & Fu (Subdue) are conceptual and do not have to be confined to the restraints of a particular technique

Yes, I'd agree with that! They are not techniques.
They are not hand or arm shapes or the movements made with them.
They are concepts, so how they are applied is purely down to circumstance.