  Non-symmorphic symmetries protect Dirac line nodes in square net materials.
This phenomenon has been most prominently observed in ZrSiS. Here, we
systematically study the symmetry-protected nodal fermions that result from
different ways of embedding the square net into a larger unit cell.
Surprisingly, we find that a nonsymmorphic space group is not a necessary
condition for a filling enforced semimetal: symmorphic space groups can also
host nodal fermions that are enforced by band folding and electron count, that
is, a combination of a particular structural motif combined with electron
filling. We apply the results of this symmetry analysis to define an algorithm,
which we utilize to find square net materials with nodal fermions in specific
symmorphic space groups. We highlight one result of this search, the compound
ThGeSe, which has not been discussed before in the context of nodal fermions.
Finally, we discuss how band folding can impose constraints on band
connectivity beyond the connectivity of single elementary band representations.
