29 CFR § 780.313 - Piece rate basis.

The exemption provides that the employee must be paid on a piece-rate basis. To be exempt the employee must be compensated solely on piece rates during the workweek. The exemption does not apply in any workweek in which the employee is compensated on any other basis. For example, if an employee is compensated on an hourly rate for part of the week and on a piece rate for part of the week, the exemption would not be available. Also, if any pieceworker who is otherwise subject to the minimum wage provisions of the Act does not meet all the requirements set forth in this section he must be paid at least the minimum wage for each hour worked in a particular workweek, regardless of the fact he is paid on piece rate unless he is exempted by some other provision of the Act.