He prepared the tax returns filed by petitioners Randolph and Karin Lenz and their company, KCS Industries, Inc. (Pet. App. 2a). In connection with an investigation of the Lenzes and their company, the Internal Revenue Service issued summonses to Frederick directing him to produce various documents-including tax forms and worksheets-relating to the preparation of these tax returns (ibid.). Although Frederick substantially complied with the summonses, he claimed that approximately 800 pages of documents were protected by the attorney-client privilege, the work-product doctrine, or both (id. at 17a). When the United States brought this suit to enforce the summonses, the Lenzes and their company intervened (id. at 2a, 33a).