To select the most appropriate tubing length:
Firmly place the Measurement Tool horizontally over the ear with the short edge of the earhook facing toward the back of the head.
Use the Measurement Tool to select tubing length.
Choose the tubing length that is closest to the mark at the opening of the top of the ear canal. If the canal opening falls between the lines, select the shorter tubing length. For the ear in picture above, a size 3 tube would be selected.