Description Lydon Fetterolf Corydon sure has a lot of letters for a numbers firm. The group of certified public accountants provides a wide range of accounting, auditing, budgeting, business and personal financial planning, computer, pension and profit sharing, and tax planning and preparation services in the Washington, DC area. Clients include corporations; associations; estates and trusts; and not-for-profit organizations; as well as professionals such as engineers, film producers, and physicians. The firm was founded in 1977 by managing partner Thomas Lydon.