Negative space with respect to buccal corridors is an indication for

The transverse dimension of the smile is also referred to as “transverse dental projection”. Lateral negative space is the buccal corridor between the posterior teeth and the corner of the mouth in smiling. Orthodontists refer to buccal corridors as “negative” spaces to be eliminated by transverse maxillary expansion. Orthodontic expansion and widening of collapsed arch form can improve the appearance of the smile dramatically by decreasing the size of the buccal corridors and improving the transverse smile dimension. The transverse smile dimension (and the buccal corridor width) is related to the lateral projection of the premolars and the molars into the buccal corridors. The wider the arch form in the premolar area, the greater is the amount of the buccal corridor that is filled. Graber Vanarsdall Vig, orthodontic current principles and techniques 6th  edition page 273