A commercial lease is the largest and least reversible contract most small businesses ever sign, and it is signed by people who read it once, quickly, in a hurry to open. This article explains what is actually in that document: how rentable square footage differs from what you can use, how a triple net structure moves costs the tenant never priced, what a personal guaranty really commits, and why the tenant improvement allowance is usually the most negotiable number on the page. It walks through the clauses that decide whether a business survives a bad year — assignment and subletting, exclusive use, co-tenancy, relocation, demolition, and holdover — and shows how each one reads to a landlord and to a tenant. Separate sections cover what happens on default, how mitigation actually works, and the three real exits from a lease that has outlived the business. Federal overlays that surprise tenants are covered, including accessibility obligations, environmental liability, and what a bankruptcy filing does to a lease. Worked numbers and concrete examples run throughout, and every authority is linked to its text.