In engineering design, a function/means tree (a.k.a. function-means tree or F/M tree) is a method for functional decomposition

Functional decomposition

Functional decomposition refers broadly to the process of resolving a functional relationship into its constituent parts in such a way that the original function can be reconstructed from those parts by function composition...

and concept generation. At the top level main functions are identified. Under each function, a means (or solution element) is attached. Alternative solution elements can also be attached. Each means is in turn decomposed into functions with means attached to each of them. A well elaborated function means tree span a design space where all concepts under consideration are represented. Requirements can be attached to functions.

In addition to the product level requirements, there might be requirements on sub functions that may be a consequence of means at a higher level. The function means tree is a tool that can aid in the creative part of the design process. It can also be a tool for mapping requirements to parts in a design. This requires that there is also a mapping between means and parts in the product architecture.