a line on a last running from the featherline of the toe up the center of the instep, over any vent, the cone, and any thimble, then down the center of the heel to the featherline again
Modern lasts are not symmetrical, so the location of the main axis through the toe is open to some interpretation. Some sources prescribe drawing through the center of where the tip of the second toe will lie in the shoe. In practice, the main axis often determines the line of symmetry for symmetric patterns, so appearance and sufficient lasting allowance affect where it’s drawn.