  One of the most striking signatures of Weyl fermions is their surface Fermi
arcs. Less known is that Fermi arcs can also be localized at internal twin
boundaries where two Weyl materials of opposite chirality meet. In this work,
we derive constraints on the topology and connectivity of these "internal Fermi
arcs." We show that internal Fermi arcs can exhibit transport signatures and
propose two probes: quantum oscillations and a quantized chiral magnetic
current. We propose merohedrally twinned B20 materials as candidates to host
internal Fermi arcs, verified through both model and ab initio calculations.
Our theoretical investigation sheds lights on the topological features and
motivates experimental studies into the intriguing physics of internal Fermi
arcs.
