Tate-Ochi is a Japanese term that refers to occurrences of vertical lines in vintage denim, which is also referred to as slubs; an industry term referring to a thick or heavy place in the yarn.
Old technology spinning frames produced yarn with unevenness. Modern yarn spinning technology produces very few slubs unless special devices are used in spinning to “engineer” them back into the yarn. Engineered slub yarns are quite common in denim today.
Regardless of how the slubs are created, the result is that the indigo fades the most where the thread is thickest. This creates a white or severely faded thread of several centimetres along a single vertical indigo thread.