  In 1988, Knabe found a "finite-size criterion" to determine whether a
frustration-free quantum spin chain with periodic boundary conditions is
uniformly gapped in the thermodynamic limit. The criterion provides a threshold
for the spectral gap at a finite system size such that, if the threshold is
exceeded for a fixed system size, then the chain with periodic boundary
conditions is uniformly gapped.
  We extend Knabe's result to frustration-free spin chains equipped with open
boundary conditions. We now obtain two finite-size criteria: The first one is
identical to Knabe's criterion and we interpret it as a bulk criterion. The
second one controls the spectral gaps at smaller system sizes and can be
interpreted as a new edge criterion. Heuristically, it excludes the presence of
thermodynamically gapless excitations living near the edge.
