In manga and anime, taresen タレ線, "dripping lines," or tate-sen 縦線, "vertical lines," refers to parallel vertical lines drawn on the background or on a character's face when the character feels down, feels sad, feels disgusted, among other negative feelings.
It's also spelled taresen 垂れ線, because it "drips," tareru 垂れる.
(sound effect used when a character suddenly feels down.)
In anime, these are often rendered blue, but blue lines can also be used in a number of other ways. See the article on Blue Lines for a complete list of meanings.
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