  An electron localization measure was originally introduced to characterize
chemical bond structures in molecules. Recently, a nucleon localization based
on Hartree-Fock densities has been introduced to investigate $\alpha$-cluster
structures in light nuclei. Compared to the local nucleonic densities, the
nucleon localization function has been shown to be an excellent indicator of
shell effects and cluster correlations. Using the spatial nucleon localization
measure, we investigate the emergence of fragments in fissioning heavy nuclei.
To illustrate basic concepts of nucleon localization, we employ the
self-consistent energy density functional method with a quantified energy
density functional optimized for fission studies. We study the particle
densities and spatial nucleon localization distributions along the fission
pathways of $^{264}$Fm, $^{232}$Th and $^{240}$Pu. We demonstrate that the
fission fragments are formed fairly early in the evolution, well before
scission. We illustrate the usefulness of the localization measure by showing
how the hyperdeformed state of $^{232}$Th can be understood in terms of a
quasimolecular state made of $^{132}$Sn and $^{100}$Zr fragments. Compared to
nucleonic distributions, the nucleon localization function more effectively
quantifies nucleonic clustering: its characteristic oscillating pattern, traced
back to shell effects, is a clear fingerprint of cluster/fragment
configurations. This is of particular interest for studies of fragment
formation and fragment identification in fissioning nuclei.
