Unless you're a beginner, your workouts are most likely divvied up by body part, and that means using multiple exercises to hit a specific muscle group from various angles in hopes of maximizing growth. That approach works well most of the time, but the trapezius muscle located on your backside presents an unusual dilemma.
Who, after all, says, "It's trapezius day"? No one I ever met.
What makes the trapezius so different is that the diamond-shaped muscle has three regions—the upper, middle, and lower —and each has a different function. Even though the trapezius is a single muscle, trainees often work those regions on separate training days. Hence the confusion, so let's separate the functions of each of the traps' regions to better determine when to train them.
When you think of big traps, you're probably thinking of the upper region between your neck and shoulders. When you elevate your scapula (shoulder blades) or upwardly rotate them, the upper region is worked the hardest. This is the same motion someone does when they shrug their shoulders in everyday life. Besides shrugs, many delt exercises including lateral raises and upright rows effectively hit the upper traps quite directly, so it makes sense to finish off with single-joint shrug movements. So, it makes sense to train the upper traps on shoulder day.
The middle portions of the trapezii primarily pull the shoulder blades together, which is called retraction. You mimic this in the gym when doing seated cable rows. (And if you're not pinching your shoulder blades together during rowing motions, you're shortchanging your middle traps!) Rows are commonly done on back day, but the movement hits other muscle groups as well. You can better isolate the middle trapezius doing seated cable rows in which your arms remain straight throughout (called a straight-arm seated cable row). Simply pinch your shoulder blades together and release, so the range of motion is pretty small. For middle traps, your best training day is with back.
To complicate matters even further, let's move on to the lower traps. Their function is to rotate the shoulder blades downward, such as during the initiation phase of pull-downs, another back exercise. In fact, you can effectively isolate the lower traps by doing just the first few inches of a pull-down by keeping your arms straight. Those lower fibers are also engaged when doing prone front raises on an incline bench, and that's considered a shoulder exercise! Even snatches, a whole-body Olympic move, engage the lower-trap fibers.
So where do traps belong in your workout split? It depends. Unless you train back and shoulders together on the same day, you're most likely hitting different regions of the muscle in different body-part workouts, and that's perfectly fine. Just know that to target it completely, you'll need a variety of motions. Too often, trainers might think shrugs are enough. Now you know they aren't.