The side slit of a qipao reads as a style detail, the sort of thing a designer decides in a sketch. It is better understood as a release joint. Take a long garment cut from one continuous length of cloth, shape it to the waist so that it stops being a tube, and the hem becomes narrower than the stride requires. Something has to give. Either the lower half stays a separate piece, or you cut an opening in the side seam and let the two panels move independently.
Once you see the slit that way, its height, its reinforcement and its placement stop looking arbitrary. Every one of them is set by how far a leg has to move.
A loom width decides the pattern
Traditional looms produce cloth in a fixed width, often somewhere between seventy and ninety centimetres for handwoven silk or cotton. A garment that uses the full width with as little cutting as possible has no shoulder seam and no separate sleeve head: the fabric folds over the shoulder, and the shape comes from the side seams. That is the same principle behind the rectangle that became a robe, where the belt did the shaping that tailoring does today.
The qipao keeps the one-piece length but adds something the older rectangle did not have: a fitted waist. Shaping the side seam inward means the circumference at the hip is smaller than the circumference at the chest, and the panels below the hip draw toward each other rather than hanging straight. A walking stride needs roughly half a metre of clearance between the two feet at their widest point. A narrow tube cannot supply it. The slit supplies it by allowing the front and back panels to separate at exactly the place where the leg needs room, while keeping the silhouette continuous when the wearer stands still.
The slit is a hinge, not a hole
Cutting a seam open creates a stress concentration at the point where the cut ends. Every step loads that one spot: the two panels pull apart, and the force concentrates at the top of the opening rather than spreading along the seam. A well-made slit is therefore reinforced right at its end, with a short bar of dense stitching, a small triangular gusset, or a hidden facing that carries the load into the surrounding cloth. The top of the opening, not the edge of the fabric, is the part that fails first.
The height of the slit is a direct trade. A low opening is enough for ordinary walking and keeps the garment closed when seated; a higher one allows a longer stride or a stair, and exposes more of the leg. There is no abstract correct value. It is a measurement of the motion the garment is being asked to permit, and it is why ceremonial versions and working versions of the same cut can look quite different.
The collar and the closure carry the load
Because the body of the garment is one piece, the opening has to be somewhere else. The qipao puts it on the diagonal across the chest and up into a standing collar, closed with frog buttons: narrow cords made from the same cloth, knotted into a loop and a ball, sewn on by hand. A cord fastening is flexible, replaceable with the same material, and adds no hard point that would abrade silk. The direction of the diagonal opening continues the older logic of the crossed collar, which settled on one overlap direction for practical reasons long before it became a convention.
The standing collar does structural work too. It stiffens the top edge of a garment that otherwise has no shoulder seam to anchor to, so the whole front hangs from a defined line. Modern shirt collars solve a similar problem with a hidden third layer, as described in why a shirt collar stays flat; here the stiffness comes from the cut and the number of layers of cloth, not from a fused interlining.
What changed when tailoring arrived
Once darts, set-in sleeves and zips were available, a fitted garment no longer needed to be one continuous panel. A skirt could be cut with enough sweep to walk in, or a dress could be shaped in three dimensions from several pieces, and the slit became an option rather than a necessity. What survived was the silhouette: a straight line from shoulder to hem, a high collar, a diagonal closure.
So the slit is a fossil of a construction method. It records the moment a woven width of cloth was asked to behave like a fitted garment, and the answer was not to add more cloth but to remove a narrow line of it.