James Perozzo (1994) The complete guide to electronics troubleshooting. p. 72
William H. Von Alven (1964) Reliability engineering explains: "Functional block diagrams show functional sequences and signal paths, and items which are wired in parallel are drawn in parallel" (p. 286)
Harry H. Goode, Robert Engel Machol (1957) System engineering: an introduction to the design of large-scale systems. p. 306 /wiki/Harry_H._Goode