For some linear mechanical systems, which can be written as a dyadic transfer function matrix (DTM), MIMO controller design can be reduced to a number of SISO designs. Internal stability of the SISO designs guarantees stability of the resulting MIMO controller. By relating the insights from modal analysis to system theory, necessary and sufficient conditions are derived for linear mechanical systems to have the DTM property. The main theorems are illustrated on an active suspension design of an agricultural spray boom. (C) 2003 Elsevier Ltd. All rights reserved.