Yale law professor Ian Ayres explains the distinction between default and mandatory rules in contract law, part of his "A Law Student's Toolkit" series aimed at incoming law students. Default rules are the terms that apply unless parties specify otherwise in their agreement, while mandatory rules are fixed requirements that cannot be waived or altered by contract. Ayres works through examples showing how courts decide which category a given rule falls into, and why the difference matters for drafting and interpreting agreements. The fourteen minute lecture builds vocabulary students need before their first contracts class, treating the default and mandatory divide as one of the basic conceptual tools of legal reasoning rather than a narrow technical point.