Reference lines (also referred to as leading lines), that is, lines between the reference signs (for example, reference numerals) and the details referred to, may be straight or curved and should be as short as possible. They must originate in the immediate proximity of the reference sign and extend to the feature indicated. Reference lines for certain reference signs may be omitted. Reference signs of this type, which are not connected to anything, will then indicate the surface or cross-section on which they are placed. In such cases the reference sign may be underlined to make it clear that the line has not been left out by mistake. Reference lines must be executed in the same way as other lines in the drawing.
PCT AG I 5.145