In Part 2 of ‘Endangerment 101,' we tackle the so-called "Tailoring Rule" (TR). What is it? That's a timely question, and the answer lies at the heart of the endangerment finding. The TR was unconsciously conceived on April 2, 2007, when the Supreme Court ruled on Massachusetts v. EPA. In that opinion, one searches in vain for any hints that the majority understood the regulatory maelstrom that inexorably ensues once endangerment is triggered.