D leased an office in P's building for a term of five years expiring on October 31, 1976. Negotiations between the parties for a two-year extension of the lease proved to be unsuccessful, and on November 16, 1976, D advised P in writing that it would vacate the premises on November 30, 1976. There is no question that D did vacate no later than the end of the first week of December 1976. P attempted to relet the premises but was unable to do so until April 1977. P sued D seeking recovery of the monthly rent for December 1976 and January through March 1977. D concedes its liability for the December rent. D claims that its mid-November notice was effective as of December 31, 1976, shielding it from any liability for rent thereafter. The trial judge awarded P rent for all four months. D appealed.