Hey @Ayush Srivastava, you are measuring using reference points on the face. At 3:55 in the demo above, Stan calls these reference points "units" and explains how to use them. A unit could be the whole head -- which is why you will hear artists reference the height of an average body as "7 and a half heads tall" -- or the even the length of the nose for example. You can use what makes the most sense to you. In the video, Stan walks you through how he thinks about it, which can be really helpful when trying to understand this concept. Give it a listen and let me know if that makes sense!