A commercial lease is two legal instruments fused into one document: a conveyance of an estate in land and a long-term commercial contract, and nearly every hard question in leasing comes from the friction between those two identities. This UpLaw guide walks the entire instrument, from the letter of intent through the rent structure, the measurement of the premises and the load factor that quietly inflates it, the operating-expense and CAM pass-throughs and the audit rights that police them, options to renew and expand, use and exclusivity and co-tenancy, assignment and subletting and the commercial-reasonableness standard of Kendall v. Ernest Pestana, the allocation of maintenance and building systems, tenant improvement allowances and restoration, casualty and condemnation and insurance, the SNDA and the estoppel certificate, personal and good-guy guaranties, and the default-and-remedies machinery, including the landlord's duty to mitigate after Sommer v. Kridel and its commercial descendants. Landlord-tenant law is state law, so the guide teaches the framework and the majority rule, flags the states that depart from it, and tells you where to check your own. A running hypothetical -- Marlow and Finch, a coffee roaster negotiating a ten-year triple-net retail lease -- carries the doctrine into the numbers, with worked math on breakpoints, load factors, expense caps, and the Section 502(b)(6) bankruptcy damage cap. It is written for the tenant and the landlord alike, because the same clause reads differently depending on which side of the table you sit.