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. Example as shown in Fig. 12 and Fig.13, reference number marked with underline 1204.