  No natural principle is currently known to be strictly between the arithmetic
comprehension axiom (ACA) and Ramsey's theorem for pairs (RT^2_2) in reverse
mathematics. The tree theorem for pairs (TT^2_2) is however a good candidate.
The tree theorem states that for every finite coloring over tuples of
comparable nodes in the full binary tree, there is a monochromatic subtree
isomorphic to the full tree. The principle TT^2_2 is known to lie between ACA
and RT^2_2 over RCA, but its exact strength remains open. In this paper, we
prove that RT^2_2 together with weak K\"onig's lemma (WKL) does not imply
TT^2_2, thereby answering a question of Montalban. This separation is a case in
point of the method of Lerman, Solomon and Towsner for designing a
computability-theoretic property which discriminates between two statements in
reverse mathematics. We therefore put the emphasis on the different steps
leading to this separation in order to serve as a tutorial for separating
principles in reverse mathematics.
