References - Effective Traffic Riding (British Cycling), Cyclecraft
There are two cycling positions - primary position ("taking the lane") and secondary position. These positions are relative to the moving traffic lane:
a moving traffic lane - that part of the carriageway along which through traffic is moving at the present time. It is a dynamic concept, changing continually with place and traffic conditions. A moving traffic lane meanders past parked vehicles and other obstructions, and does not necessarily coincide with any markings on the surface.
The primary position ("taking the lane") is in the centre of the moving traffic lane (the blue arrow in the picture below).
Adopting the primary position makes it easier for drivers to see you and also prevents drivers from overtaking you. It should be your normal riding position.
The secondary position is about 1 metre to the left (right in the US, etc) of the moving traffic lane but no closer than 0.5 metres to the edge of the road. The secondary position should be used when it will help others without impairing your own safety (e.g. wide roads where it is safe for drivers behind to overtake you).