37 CFR § 1.84 (q)
In Patent or Utility Patent Drawings, the parts of the product or any feature or process are designated by a number or alphanumeric. These numbers are said as Reference numbers in Patent Drawings.
Reference number lead lines are those lines between the reference characters and the details referred to.
Lead lines may be straight or curved and should be as short as possible.
Lead lines must not cross each other.
They must originate in the immediate proximity of the reference character and extend to the feature indicated.
Lead lines are required for each reference character except for those which indicate the surface or cross-section on which they are placed. Such a reference character must be underlined to make it clear that a lead line has not been left out by mistake. For example, as shown in Fig. 12 and Fig.13, the reference number is marked with underline 1204.
