  Jet substructure quantities are measured using jets groomed with the
soft-drop grooming procedure in dijet events from 32.9 fb$^{-1}$ of $pp$
collisions collected with the ATLAS detector at $\sqrt{s} = 13$ TeV. These
observables are sensitive to a wide range of QCD phenomena. Some observables,
such as the jet mass and opening angle between the two subjets which pass the
soft-drop condition, can be described by a high-order (resummed) series in the
strong coupling constant $\alpha_S$. Other observables, such as the momentum
sharing between the two subjets, are nearly independent of $\alpha_S$. These
observables can be constructed using all interacting particles or using only
charged particles reconstructed in the inner tracking detectors. Track-based
versions of these observables are not collinear safe, but are measured more
precisely, and universal non-perturbative functions can absorb the collinear
singularities. The unfolded data are directly compared with QCD calculations
and hadron-level Monte Carlo simulations. The measurements are performed in
different pseudorapidity regions, which are then used to extract quark and
gluon jet shapes using the predicted quark and gluon fractions in each region.
All of the parton shower and analytical calculations provide an excellent
description of the data in most regions of phase space.
