When the image is split into two halves with a line. Several uses for this include comparing a character's different forms; placing two characters side by side in two columns (for more columns, see column lineup); or a technique used to convey two clashing characters.
See also
Column lineup - character lineup split into different columns or rows
Split crop - only one side of the character is shown
Split theme - for images composed as one character but with two different halves
