This part of ISO 8370 specifies a dynamic method of determining the location of the pitch zone of a single V-belt in a single pulley groove or of a joined V-belt in a multiple pulley which is expressed as an effective line differential be (see figures 1 and 2), or of a single V-belt in a single pulley groove which is expressed as a datum line differential bd (see figure 3). A hexagonal belt can be treated as a single V-belt.