The threshold of a lattice door is usually embedded between two columns, which is as long as the width of the bay. A door bolt is often installed on the bay, and there will be a small wooden or stone threshold two or three inches high under the small door.

this is the ancient way to threshold ancient buildings.

three inches refers to the upper, middle and lower three dantian fields. Out of the "Huangting Neijing Lingtai Chapter": "Lingtai is gloomy and looks at the yellow field, and the three-inch room has ups and downs.