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For complicated systems, it is sometimes beneficial to illustrate the various functions as a tree structure called a function tree. A function tree is a hierarchical functional breakdown structure starting with a system function or a system mission and illustrating the corresponding necessary functions on lower levels of indenture. The function tree is created by asking how an already established function is accomplished. This is repeated until functions on the lowest level are reached. The diagram may also be developed in the opposite direction by asking why a function is necessary. This is repeated until functions on the system level are reached. Function trees may be represented in many different ways. An example is shown in Figure 2.4.

Figure 2.4 Function tree (generic).

A lower level function may be required by a number of main functions and may therefore appear several places in the function tree.