We tend to think in terms of decades because they offer a shorthand way of describing the look, sound and style of a particular period.
The truth, however, is that time moves in uneven increments. The late 80s have very different feel from the early 80s, for example.
Probably the starkest peacetime shift in culture happened during the 1960s. Compare the stylish, sauve, Henry Mancini-themed films of the early 60s with the long-haired look at the end of the decade.
The Thomas Crown Affair is one of those films that takes place at the frontier of two differing eras. Steve McQueen is generally dapper, a man of means who dressed the part of the establishment type, but under his fitted shirts he's also wearing a gold chain with a large medallion - a very 70s stereotype.
Faye Dunaway is more nuanced, with her outfit conjuring both early 60s business style and the later bra-less 70s look - complete with slightly flared slacks.
This seems like a long digression for a movie review, but it's important because The Thomas Crown Affair is very much a piece of a specific time. The "perfect" bank caper is dependent on the technology of the time, and the sums of money stolen will - after the end of the gold standard and a decade of stagflation - seem ludicrously small.
Indeed, bank robberies still happen, but it's no longer possible to for a the same type of profit because money is worth so much less. In a world where nice homes cost $20,000, getting away with $100,000 is quite an achievement.
The heart of the The Thomas Crown Affair is the dueling match between McQueen and Dunaway, and there are few actors today who could pull it off. The interplay is a joy to watch.
Another fun part of the film is its experimentation with various split screens, which sometimes gets annoying, but is absolutely a part of the time. I recall the Disney revival of The Love Bug used this to great effect.
The transition from 60s to 70s was before my time, and as someone who regards 70s fashion as an unmitigated disaster (well, except for the bra-less thing), movies like this are fun to watch.