This video describes what the “phase dots” are that you often see adjacent to windings of a transformer. It discusses how these dots are used in certain circuits to establish a desired phase relationship between the signals on the various windings. A couple of tips are given regarding how to determine the phasing relationship on homebrew transformers. Finally, measurements are made with an oscilloscope on a trifilar wound transformer to see the phase relationship between the signals on each of the windings with respect to the phasing dots.
Basics of Phase Dots on Transformer Windings – [Link]