Borderline (境界線/Kyoukai-sen) is the basic ability of all shinki. It is performed by extending the index and middle fingers into a halberd shape and drawing a line between "self" and "other". The technique is typically accompanied by shouting "A line!" (一線/Issen). The barrier it creates cannot be crossed by Ayakashi.
The borderline ability is a necessity when performing an ablution (禊/Misogi). Three shinki must create intersecting borderlines in a triangular shape around the shinki being purified.
The borderline can be wielded as a weapon when drawn in the air, and manifests as blades of light that can cut through objects.
It is stated that shinki spells do not work on gods, however Borderline seems to be a special case, as both its barrier and weapon forms have been shown affecting gods.