Full Idea
The principle of Supplementation says that y is properly part of x, only if a z exists that 'makes up the difference' between them. [note: that is, z is disjoint from y and sums with y to form x]
Gist of Idea
y is only a proper part of x if there is a z which 'makes up the difference' between them
Source
Stephen Yablo (Aboutness [2014], 03.2)
Book Reference
Yablo,Stephen: 'Aboutness' [Princeton 2014], p.47