In early 1962, Edsger W. Dijkstra presented an analogy of a classroom teacher calling upon one of her pupils. By doing so, he conveyed some subtleties of “dynamic memory management” and “concurrent process behavior”.
To clarify his step-wise program composition, Dijkstra used terms like “program layers” and “levels of abstraction” in his `Notes on Structured Programming' [1, Chapter 1]. These terms were not well defined as Dijkstra conceded later. As a result, Dijkstra's exposition — although extremely rich in content — was difficult to understand completely. See e.g. Denning's clarifying article on Structured Programming  or my interview with Liskov who mentions Parnas in this regard .
In his 1972 `Notes on Structured Programming' [1, Chapter 1], Dijkstra introduced a methodology called step-wise program composition. He explained how to compose a program in minute steps, deciding each time as little as possible so that the correctness of each step is obvious.
Although he was a theoretical physicist by education, Dijkstra had been exposed to his mother's mathematical thinking during most of his teens. This observation helps explain why he continued to seek general mathematical arguments in his 1972 `Notes on Structured Programming'.
Dahl, Dijkstra, and Hoare's book "Structured Programming" was published in June 1972. Dijkstra wrote the first chapter, `Notes on Structured Programming', in which he introduced several new ideas. One of Dijkstra's main points was that mathematical techniques for small demonstration programs do not necessarily work for programs a thousand times larger.
“I still remember it well, the day my future husband entered my life”, Ria Debets-Dijkstra recalls. “He was a good-looking man, 20 years of age. He entered our Computing Department with a cane!” . The Computing Department was part of the newly founded Mathematical Centre in Amsterdam. Ria Debets-Dijkstra had already been working there for two years before she saw Edsger Dijkstra on that eventful day in 1951. Dijkstra officially joined the Computing Department in March of the following year.
Dijkstra contributed greatly to introducing hierarchical systems into computing. Already in his very first report of 1953 as employee of the Mathematical Centre in Amsterdam, Dijkstra introduced what he would later call separate levels of concern . In 1959–60, he acquired fame after reformulating the problem of ALGOL translation as an hierarchical problem. His intermediate machine-independent stack-based language was, in hindsight, a precursor to the now widely used virtual machine; see chapter 3 in my book The Dawn of Software Engineering .
On Tuesday 12 January 1982, Dijkstra wrote a letter to Dr. A. Caplin explaining why he had his reservations about the language APL (A Programming Language). Dijkstra did this in response to an earlier letter that he had received from Caplin, a letter in which Caplin asked whether Dijkstra favored APL and, if not, why not. In Caplin's words:
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