What is “usury” in medieval Christian teaching?

The story behind the answer

In medieval Christian doctrine, any payment beyond the principal of a loan was treated as a sin. Drawing on Scripture (e.g., prohibitions on interest to neighbors) and Aristotle’s claim that “money is barren,” theologians like Aquinas argued that charging for the mere passage of time was unjust “double payment.” Church councils, such as the Third Lateran Council (1179), condemned the practice and threatened usurers with penalties.

Yet towns and trade ran on credit, so workarounds flourished: profit‑sharing partnerships, rent-like “census” contracts, and compensation for actual losses (damnum emergens) or foregone gain (lucrum cessans). Where Christians were barred from interest-taking, Jewish and Italian lenders filled the niche, sometimes provoking resentment. Late scholastics gradually distinguished licit interest from abusive exaction, a shift that opened the door to early modern banking.