Cardiac Axis 

Look at leads I, II, aVF

Leads I, II, aVF "look" in the direction of the normal vector (down to the left)

Since the leads look in the direction of the vector... the inflection should be positive (most of the QRS going up)

Normal Cardiac Axis

The net directional movement of electricity is toward the LV (normally)

When things (ischemia, hypertrophy) affect conductivity... the vector can change

Left Axis Deviation

Lead II differentiates pathologic from physiologic LAD

(Isoelectric in lead II = physiologic)

Right Axis Deviation

Extreme Right Axis Deviation

Thumb Rule

Your left thumb = Lead I

Your right thumb = Lead aVf

Two thumbs up = Normal

Left thumb up and Right thumb down = LAD

Left thumb down and R thumb up = RAD

Both thumbs down = extreme RAD