The original push/pull/legs split is extremely popular. It's effective, but it doesn't quite check all the boxes: sufficient frequency, minimal negative impact from one workout to the next, the best ratio of work-days to rest-days for recovery, and hitting everything sufficiently at least twice per week in four weekly workouts.
While you have minimal negative carryover from workout to workout, it's hard to hit every muscle twice every 7-8 days unless you do up to 5-6 workouts per week, which will be too much for most.
The way to make push/pull/legs into an optimal split is to add a fourth workout: a whole-body day. The split would become:
- Monday: Whole Body
- Tuesday: Off
- Wednesday: Legs
- Thursday: Off
- Friday: Push
- Saturday: Pull
- Sunday: Off
You can use the whole-body workout to work on strength and the other days to focus more on maximizing growth or addressing weak links in the big lifts.
For example, on Monday, you'd do a squat pattern, a press pattern, a pull/row, and a deadlift. Nothing else. Then on Wednesday, Friday, and Saturday, you'd train muscles to maximize growth, with an emphasis on the lagging muscles in the big lifts:
- Wednesday: Assistance exercises to build the squat
- Friday: Assistance exercises to build the bench press
- Saturday: Assistance exercise to build the deadlift and row (or chin-up)
This is a motivating split. Each workout has a very specific purpose, and you have two different "feels" during the week.
You can also use the whole-body day for something different than strength, like conditioning work or athletic training.