FORTUNE — In a chart posted on Twitpic Monday, Asymco‘s Horace Dediu shows that the multiple of PCs sold to Apple AAPL Macs sold has been falling steadily since it peaked in 2004 and is approaching the ratio of 1985.

Brian S Hall points out in his Smartphone Wars blog that 1985 was the year Microsoft MSFT released Windows. “It was the beginning of the end for Mac and Apple,” he writes. “Or so we thought.”

What turned the tide? My best guess is the release of iTunes for Windows in 2003 and the subsequent spread of the so-called iPod halo effect. Note also the drop in the PC-to-Mac ratio in 2007, the year the iPhone was introduced.

UPDATE: On Wednesday, Dediu filed an article about this ratio, noting that when iOS devices are included in the comparison, Windows’ advantage over Apple drops below 2 — a multiple that is likely to be reduced to parity with a year or two.

“The consequences are dire for Microsoft,” he writes. “The wiping out of any platform advantage around Windows will render it vulnerable to direct competition. This is not something it had to worry about before. Windows will have to compete not only for users, but for developer talent, investment by enterprises and the implicit goodwill it has had for more than a decade.

“It will, most importantly, have a psychological effect. Realizing that Windows is not a hegemony will unleash market forces that nobody can predict.”

Below: Dediu’s updated chart.