as the anatomical crown and root tapers towards the apex, the mesial-distal dimension decreases, and so the mean mesial-distal dimension at the marginal crest of bone, which lies approximately 2 mm apical to the cej, is smaller. the tooth has the apico-coronal distance from the marginal crest of bone to the contact point in order to increase from the much narrower mesial-distal dimension to the greater mesial-distal dimension, and this distance is partially subgingival (below the gum line) and partially supragingival (above the gun line). "running room" refers to this subgingival apico-coronal distance.