Full Idea
If my dog and the top half of my tree compose an object, this is defended under the title of 'unrestricted (universal) composition', the thesis that any two things compose something.
Gist of Idea
'Unrestricted composition' says any two things can make up a third thing
Source
Trenton Merricks (Objects and Persons [2003], §1.II)
Book Reference
Merricks,Trenton: 'Objects and Persons' [OUP 2003], p.8
A Reaction
David Lewis is cited amongst those defending this thesis. My intuition is against this thesis, because I think identity is partly dictated by nature, and is not entirely conventional. You can force an identity, but you feel the 'restriction'.