Where the words come from
The Clear It Up on there being many labour markets, the first rule of labour markets, required education as a limit on supply, technology and wage inequality, and the race between the supply of and demand for skilled labour — Principles of Economics 2e, OpenStax, licensed CC BY 4.0. https://openstax.org/books/principles-economics-2e/pages/4-1-demand-and-supply-at-work-in-labor-markets and https://openstax.org/books/principles-economics-2e/pages/15-4-income-inequality-measurement-and-causes and https://openstax.org/books/principles-economics-2e/pages/14-1-the-theory-of-labor-markets. Changes: the Clear It Up and marginal-product passages from section 14.1 folded in; the US degree figures (840,000 in 1970 against 1,894,934 in 2013-14) summarised as "more than doubled" and attributed to the United States; the US earnings-ratio figures dropped; "primary care physicians" to "GPs"; "file clerks" to "filing clerks"; "labor" to "labour" and British spelling throughout; anatomy 4: required education and technology moved from the explains into concept cards, their definitions recast from Table 4.3 ("The more required education, the lower the supply") and from section 4.1's account of technology as a substitute for low-skill and a complement to high-skill labour; "file clerks" example kept on the technology card, with 4.1's substitute-and-complement result and 15.4's widening skill gap; "retail sales vs. scientist" made "a scientist's pay and a shop assistant's pay"; the compensating-differential card and the recap are our own wording.
The question and the page layout are our own.