Proprietary software is often a company's most valuable asset, yet no single body of law protects all of it, because a program is at once a written work, a functioning machine, a confidential secret, and a licensed product. This article maps the four legal regimes that protect software—copyright, patent, trade secret, and contract—showing precisely what each one covers, where each one fails, and how counsel weave them so that one tool's gap becomes another's strength. It works through the governing statutes and the decisions that shape them, from Altai and Alice to Google v. Oracle, Vernor, Bowers, and Van Buren, following an invented port-scheduling startup from first commit to courtroom. It explains who actually owns code, why an unsigned assignment can quietly sink an acquisition, and how to fit the four regimes to a company's real technology, business model, and threats. A reader with no legal training should come away understanding not only the rules but the strategy behind them.