When there’s nobody on, yeah, that’s the positions they usually take. It shifts when runners are on base, though.
Sure, but then the asymmetry is caused by the presence of the runners. A diamond with runners on it is not symmetrical the way a diamond without is, and the distribution of the infielders responds to that. It’s like space that stops being flat as soon as some massive objects are placed in it.
There’s also the issue that with the preponderance of right-handed hitting, and hence preponderance of balls hit to the 3rd base = left half of the field, the defense usually wants to be leaning to the batter’s left.
In a nutshell: the attack isn’t symmetrical, so the defense should not be either. At least not when averaged across all the batters.
Apparently, back in the 1850s, it was standard for the then-three infielders to position themselves close to their bases, so it was symmetrical then, with the second baseman effectively in the middle of the infield (left-to right). Adding a fourth infielder in the shortstop created a different symmetrical arrangement.
Not dramatically. Neither the 2B or SS is standing on the base to hold someone on - maybe one of them makes a move to the base for a pickoff attempt. The only position player whose positioning changes much with someone on base is a 1B.
In any sense that matters, then, the answer is “always.” The modern version of baseball was only codified between 1846 and 1857.
Even then, truly “modern” baseball is more like a late 1940s start date. I know they maintain stats back to before then, but the flaky fields, flaky balls, and totally different business model make pre-WWII and post WWII utterly different sports. Wildly differing degrees of athleticism and professionalism too.
IMO, and probably a controversial O.
Many baseball fans would peg the start of “modern” baseball a little earlier than that, to around 1921. The couple of decades before that are now referred to as the “dead-ball era,” it was marked by low batting averages, offensive strategies built around “small ball” (base hits and stealing) rather than power hitting, and dominant pitching.
The dead-ball era ended pretty suddenly; factors which led to the modern game included a change in rules to mandate replacing damaged or dirty baseballs (which were harder for batters to see, and harder to hit for distance), the banning of the spitball and similar pitches which relied on “doctoring” the ball, smaller ballparks (which were easier to hit home runs out of), and the demonstrated success of slugging in one Babe Ruth.
On the other hand, one thing that would point to your “late 1940s” as being a difference is in batting averages; there are a number of players who primarily played in the 1920s and 1930s whose career batting averages (in the .340s) were pretty much unheard of by later standards*. Plus, in the AL and NL, there were nine instances between 1920 and 1941 in which a player had a season batting average of .400 or greater; no one has reached that milestone since Ted Williams in '41.
*- There are only two players who primarily played in the postwar era who hit .330 or better for their careers: Tony Gwynn (.338) and Stan Musial (.331).
Dead-ball era was a very real phenomenon. And widely acknowledged as such. No debate from me there.
And yeah, I agree my POV is minority. But the pro game entered a weird hiatus during WWII and emerged, much like the larger economy, into a new place of renewed vigor that was fundamentally different than the pre-war situation.
Then once Jackie Robinson made his history and the rest of the non-white athletes had an ever-increasing chance, we entered the “real” modern era.
That’s an even better point: the majors started to finally integrate in '47, when Robinson joined the Dodgers.