Spring bars are the tubular pins that hold the watch strap in the lugs. Simple in appearance, they are the most frequent point of failure in daily use, and when they give way, they do so without warning. From the bench, I've seen Rolex watches hit the floor because of a spring bar bought at the market for next to nothing. Material matters: high-quality stainless steel, diameter and load capacity appropriate to the weight of the case.
In the traditional version, the spring bar has two spring stems that compress with a dedicated fork or point tool, allowing insertion or removal from the lug. The operation requires practice: forcing it at an angle means scratching the lug or sending the pin flying across the bench. Anyone who works with vintage watches knows that original spring bars from the Fifties and Sixties often had thicker caliber, with greater holding tolerance, and replacement bars must be chosen with care regarding dimensions.
Drilled lugs—the so-called drilled lugs—change the approach: the through-hole allows insertion of the spring bar from the outside with a simple pin, without special tools. Aesthetically they divide collectors; technically they simplify life. On certain vintage pieces, the presence or absence of the hole is an indicator of documented production variation.
Quick-release spring bars integrate a lever mechanism or lateral pressure system that allows strap removal without tools. The concept is sound; execution varies enormously: the best have minimal play and crisp engagement; the economical ones disengage on their own or won't disengage at all. Some manufacturers offer them oriented for right-handers or left-handers, a detail that shows design attention. They don't replace the traditional spring bar in contexts where replacement is rare and retention must be maximum.
One thing I almost never see written: spring bars should be replaced periodically, like batteries. They don't exist to last thirty years. If the watch is worn every day, the spring bar should be checked every two or three years. I don't know with certainty the fatigue cycles of different manufacturers, but the principle doesn't change.