A retainer agreement is a work-for-hire contract. It falls between a one-off contract and permanent employment, which may be full-time or part-time. Its distinguishing feature is that the client or customer pays in advance for professional work to be specified later. The purpose of a retainer fee is to ensure that the employed reserves time for the client in the future when their services are needed. A retainer agreement may incorporate other contractual provisions regarding the performance of services, or the parties may potentially enter into additional contracts that define the other terms of their working relationship. A retainer fee may be paid on a fixed, pre-negotiated rate or on a variable hourly rate depending on the nature of retainer and also, the practice of the professional being retained.