Whether leased premises are used exclusively as offices must be assessed at the date of renewal of the lease. Having found that the clause on the permitted use of the leased premises was not ambiguous, since it expressly referred to the nature of the premises, namely offices, and to the activities of the tenant company, which were intellectual and administrative in nature, and that the tenant had not established that the common intention of the parties was to use the leased premises as an educational establishment, the court of appeal correctly inferred that the rent of the renewed lease was to be uncapped.