Full Idea
A simple way to think about internal relations is: if R internally relates a and b, then, if you have a and b, you thereby have R. If you have six and you have five, you thereby have six's being greater than five.
Gist of Idea
If R internally relates a and b, and you have a and b, you thereby have R
Source
John Heil (Relations [2009], 'External')
Book Reference
'Routledge Companion to Metaphysics', ed/tr. Le Poidevin/Simons etc [Routledge 2012], p.314
A Reaction
This seems to work a lot better for abstracta than for physical objects, where I am struggling to think of a parallel example. Parenthood? Temporal relations between things? Acorn and oak?
Related Idea
Idea 21351 Truthmaking is a clear example of an internal relation [Heil]