Strip miter
The gutter is cut and folded on site so the two runs meet directly, with the joint sealed from the inside. Cleanest look, fewest parts, and the strongest option when it is formed well — but the quality depends entirely on the person making the cut. A sloppy strip miter is a leak with a countdown on it.
Box miter
A prefabricated corner piece that the two runs slot into, sealed on both sides. Faster to install and more forgiving of an imperfect cut, at the cost of two seams instead of one and a slightly heavier look at the corner.
Bay miter
Any corner that is not 90 degrees — bay windows, angled additions, and the clipped corners common on newer Kansas City builds. These have to be cut to the actual measured angle. A bay miter forced to fit an approximate angle is the leak we find most often on homes under fifteen years old.
Why corners leak first
Three things stack up at a corner. Water changes direction there, so flow is turbulent rather than smooth. Two pieces of metal meet, so there is a joint that expands and contracts at slightly different rates. And the sealant in that joint is the only thing between the two — so when it goes brittle after a decade of Kansas City summers, the corner opens up.
The tell is water running down the outside of the corner during rain, or a stained streak on the fascia directly below it that never fully dries.
Repairing vs replacing a miter
Adding sealant over a failed joint is a short-term patch — new sealant will not bond to old, degraded sealant. A proper repair means opening the joint, cleaning both faces back to bare metal, and re-sealing from the inside so water pressure closes the seal rather than peeling it. Done that way it is a repair that lasts, not one you revisit next spring.