利用方框图组合分析线性系统的方法，可以使求解过程简化，它包括相加，倍乘以及积分构成连续的模型。改方法也称为线性系统的仿真（simulation）。为了进一步简化改方法，美国麻省理工学院的梅森（Mason）于20世纪50年代初提出利于线性系统的“信号流图”。

A signal-flow graph or signal-flowgraph (SFG), invented by Shannon, but often called a Mason graph after Samuel Jefferson Mason who coined the term, is a specialized flow graph, a directed graph in which nodes represent system variables, and branches (edges, arcs, or arrows) represent functional connections between pairs of nodes. Thus, signal-flow graph theory builds on that of directed graphs (also called digraphs), which includes as well that of oriented graphs. This mathematical theory of digraphs exists, of course, quite apart from its applications.