Lineup Construction
Who bats where, so the best hitters get the most trips.
How it actually works. The traditional shape puts a fast contact hitter first, a bat control hitter second, the best hitter third and the biggest power fourth. The modern shape puts the best hitter second, because the second spot comes to the plate more often over a season than the third and comes up with runners on more often than the first.
Why it is done, and when it is wrong. The honest range is worth stating: rearranging a lineup optimally rather than badly is worth a small number of runs across a whole season, which is real and is far less than fans and broadcasters believe. The two things that actually matter are getting the most plate appearances to the best hitters, which pushes them up, and not stacking hitters who make outs at the top, which wastes the innings where the order is guaranteed. Everything else is preference.
What people get wrong about it. The number nine hitter in a designated hitter league is effectively a second leadoff man, and clubs that treat it that way get an extra good at bat in front of the top of the order most times through.
At Little League and travel ball. Batting order matters much more in youth ball than in the professional game, because the gap between the best and worst hitters on a youth roster is enormous and games are short. Continuous batting orders, which many youth leagues require, change the question entirely: everybody hits, so the order is about who comes up in the innings that decide it.
Batter's Eye: look up anything in baseball