Q-Factor
This is the distance between the outside faces of your crank arms. Effectively, a larger Q factor means your pedals are further away from the bike's center line, and therefore your feet are further apart.
A narrow Q-Factor is considered more aerodynamic, but increases the risk of chaffing the inner thigh on the nose of your saddle. Likewise a wide Q factor means your knees and thighs are opened up, which may help joint comfort. Going too wide will make joints painful.
Q-factor can be increased by adding extenders, or using wider pedals.
Q-factor can only be reduced by changing the cranks for narrower ones, which increases the possibility of heel/shoe strike on the crankset or chain or front derailleur.
Note - q factor is NOT measured from the centerline because it could be different on each side.
