The transparent circle represents the position of the subtalar joint (20) when the foot is supinated, pronated and in a neutral position (e.g., joint congruity).  This position is approximately one finger width below and proximal to the medial malleolus.

Using a microwedge to measure the Primus Metatarsus Supinatus value in the Rothbarts Foot  (readings are in millimeters): F1 - Foot is pronated, F2 - Foot is placed into its anatomical neutral position, F3 - microwedge placed under the foot.

PMSv 11 - 29mm is pathognomic of Rothbarts Foot (See Interpreting PMSv)