A covenant in a lease determining whether the tenant can alienate the land, that is, whether there is any absolute prohibition or restriction against the tenant charging, assigning, underletting or parting with possession of the demised premises. Where the alienation covenant provides that the tenant cannot assign, underlet, charge or part with possession without the consent of the landlord, it is implied that the landlord's consent will not be unreasonably withheld (section 19, Landlord and Tenant Act 1927).