The following is a non-exhaustive and incomplete list of artistic conventions used in mainstream manga and their place of origin.
A white cross-shaped bandage symbol denotes pain.
A large sweat drop on the side of the face denotes a broad spectrum of emotions, usually embarrassment or exasperation.
A scribble on the cheek can show injury; it is also used in black-and-white media to denote red cheeks, i.e. blushing.
A red cheek denotes embarrassment or blushing.
A throbbing vein, sometimes comically simplified to a "+" shaped outline on the head (or occasionally other body parts, especially fists), represents anger or irritation.
A balloon dangling from one nostril (a "snot bubble") indicates sleep.
Electricity shoots out on the eyes of two characters when they are fighting.
Nosebleeds, usually caused by shocking sights - especially those with a sexual undertone.
There are many eye symbols such as love-hearts, crosses, flames, stars, and spirals.
A character suddenly falling onto the floor, usually with one or more extremities twisted above him or herself, is a typically humorous reaction to something ironic happening.
The pupils disappearing from the eyes, and the iris gaining a glass-like glare smoothness denotes loss of conscious control because of possession (ghost, demon, zombie, magic, etc).
The eyes becoming huge and perfectly round with tiny pupils and no iris and going beyond the reach of the face, plus the mouth becoming like a stretched semicircle, the point of which extends past the chin, symbolises extreme excitement.
All facial features shrinking, the nose disappearing, the character lifting off the floor and the limbs being multiplied as if moving very fast symbolizes panic; if the same but with larger facial features it symbolises comic rage.
Tear drops everywhere indicate intense joy or sadness.
An ellipsis appearing over a character's head indicates a silence, implying that something is going unsaid.
Eye shapes and sizes are often symbolically used to represent the character. For instance, bigger eyes will usually symbolize beauty, innocence, or purity, while smaller, more narrow eyes typically represent coldness and/or evil.
More often than not, character colorizations tend to represent the character in some way. A more subdued character will be colored with lighter tones, while a flamboyant character will be done in bright tones. Similarly, villains are often colored in darker tones, while colder character will be given neutral tones (black, white, grey, etc.).
An odd white shape (more often than not, something close to a mushroom) that appears during an exhale represents a sigh of awkward relief or depression.
Completely blackened eyes (shadowed) indicates a vengeful or deep anger. It could also indicate that someone's being sort of a wise-guy, grinning.
Characters push their index fingers together when admitting a secret or telling the truth to another.
A character's eyes are shadowed regardless of the lighting in the room when they become angry, upset, something is wrong with them, or they are emotionally hurt.
The anime character's eyes turn into two thick half-circles, conveying a cute, delighted look.
Face expressions change depending on their mood, and can look from apple shaped to a more subtle carrot shape.
Parallel vertical lines with dark shading over the head or under the eye may represent mortification or horror. If the lines are wavy, it may represent disgust.
A wavy ghost coming out of the mouth is often a comical representation of depression or mortification.
Cherry blossoms indicate a sweet or beautiful moment. This is a reference to Mono no Aware.
A flower blossom falling off its stem may indicate death or, more commonly, sex.
A fang peeking from the corner of the mouth indicates mischief or feistiness. (Unless, of course, the character has fangs normally).
A cat mouth (like a number "3" rotated 90° clockwise) replacing the character's normal mouth, and usually accompanied by larger eyes may also represent mischief or feistiness.
Unbound hair may represent freedom, while hair that is tied back may represent some form of either literal, figurative or emotional enslavement of some kind.
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