With the right technique, your toothbrush does a pretty good job cleaning the front and back sides of your teeth.
However, it is not the best at cleaning in between your teeth. Floss or interdental brushes are essential to removing all of the bacteria from all sides of your teeth.
Whether you use floss or interdental brushes, comes down the size of the gaps between your teeth.
With interdental brushes, always use the largest size brush that fits between the gaps.