A leader line is a line that establishes a connection between a graphical representation of an item and some text. A leader points to a bit of our drawing and says:
‘Oi – Look Here’ and ‘Read This!’.
The leader line itself should be a continuous Thin line (see this post on Linetype Definitions). A leader line also has a terminator and some text. A leader line may have a reference line under the text.
The British technical drawing standards give us four different types of terminators to use with our leader lines.
An arrow terminator is used to point to an edge of an item. The dot is used to point to a face. The Architectural tick can be used for referring to multiple parallel edges. The final type of line has no terminator, and is used for pointing at dimension lines or lines of Symmetry.
Leader lines can have multiple segments and you can use one annotation to reference multiple faces and edges.
You can reference multiple parallel edges or faces with one leader.
It is recommended that you draw your leader lines at an angle to the rest of the drawing – i.e. Not parallel to the drawing itself. Increments of 15° are advised.
Finally, your Instruction text can be centred to the reference line, or it can sit above the reference line. The text should be a least twice the line thickness above the reference line. The text should never touch the reference line itself.