noun
underline; side line
A line drawn alongside text for emphasis, equivalent to underlining in horizontal writing. In vertical Japanese text, the line is drawn to the right (or left) of characters.
重要な箇所に傍線を引いてください。
Please draw a side line next to the important parts.
縦書きの文書では、傍線は文字の右側に引かれる。
In vertically written documents, side lines are drawn to the right of the characters.