In this 1970 paper, presented at the Western Conference of the Institute of Electrical and Electronics Engineers (IEEE) in 1970, author Winston Royce sets forth the development steps that came to be known as Waterfall.  Unfortunately, it was a "straw man" version of Waterfall in his Figure 2, a version without iteration and feedback loops, that came to be popularized as "Waterfall". Royce himself was quite emphatic in his graphical portrayal of "modified Waterfall" in Figures 3 and 4 that to be successful, there had to be "feedback" from a successor phase back to a predecessor phase, and that ideally the feedback could be "contained" to pair-wise successive steps, even this was not always possible, as sometimes feedback might proceed, say, from Testing all the way back to Requirements.