The percolation threshold is the filler content at which a dispersed phase first forms a connected path across a matrix, so that a transport property switches from that of the matrix to that of the filler ^[1]. Below the threshold the filler sits in isolated clusters and conductivity stays negligible; at the threshold the largest cluster spans the sample and the property rises by orders of magnitude ^[1]. For spherical fillers the threshold is predicted near sixteen volume percent, and it falls sharply as the aspect ratio of the filler rises, which is why nanotubes and graphene reach the transition below one percent ^[2]. The measured value depends on dispersion and on alignment, since agglomerated or oriented fillers percolate at loadings different from well-dispersed isotropic ones ^[3]. The transition is sharp in the thermodynamic limit and rounded in a finite specimen ^[1].