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Full Idea
There is a necessary connection between the instantiating of a structural universal by the whole and the instantiating of other universals by its parts. We can call the relation 'involvement', a nondescript word.
Gist of Idea
Structural universals have a necessary connection to the universals forming its parts
Source
David Lewis (Against Structural Universals [1986], 'What are')
Book Ref
Lewis,David: 'Papers in Metaphysics and Epistemology' [CUP 1999], p.81
A Reaction
In the case of a shape, I suppose the composing 'universals' [dunno what they are] will all be essential to the shape - that is, part of the very nature of the thing, loss of which would destroy the identity.