Leica M Instruction Manual page 51

Superimposed image method (double image)
In a portrait, for example, aim the metering field at the eye and turn
the distance setting ring on the lens until the contours in the metering
field are brought into line. Then choose the subject detail.
Out of focus In focus
Split image method
When taking photographs of architecture, for example, aim the range
finder metering field at the vertical edge or another clearly defined
vertical line and turn the distance setting ring on the lens until the
contours of the edge or line can be seen at the limits of the metering
field with no misalignment. Then choose the subject detail.
Out of focus
176
In focus