The ZERO WIDTH JOINER used in many emoji sequences is Unicode code point U+200D.
A zero width joiner, abbreviated ZWJ, is an invisible formatting character. In emoji sequences, it can connect two or more characters so that a compatible font displays them as one combined pictograph. Examples include family groupings, professions, and some gender-specific or direction-specific emoji.
The joiner itself has no visible width and is not an emoji picture. Its effect depends on a sequence supported by the platform’s emoji font. If a system does not support a particular sequence, users may see separate emoji or a fallback symbol.
ZWJ is easy to confuse with a variation selector. U+FE0F requests emoji-style presentation, whereas U+200D joins characters. Both may appear in the same sequence, but they perform different technical roles.