148 x 12mm

This dimension is typically referred to as Boost which was developed as a means to improve the stiffness/strength of 29"" mountain wheels. The hub flanges are slightly wider which act to increase wheel strength since the spokes are on a greater angle to the rim. With the larger diameter of 29"" wheels, many riders felt they weren't stiff enough so this Boost specification was introduced but then 27.5"" wheels adopted it as well to increase wheel stiffness which ended up putting 29"" wheels right back in the same position relatively speaking.

IMPORTANT: Bicycle frames need to have Boost specific spacing of 148mm x 12mm. The 142mm x 12mm predecessor is commonly confused since they are so close in size but significant damage or injury can result from mixing and matching these two dimensions.