Notdef glyph

Notdef glyph is a special character that indicates that the current font does not contain a visual representation (glyph) for the character that should have been displayed. In other words, the Unicode code point has no associated graphical representation.

Usage

It is used to visually indicate that a Unicode character has no associated glyph in the current typeface. The notdef glyph is thus displayed instead to signal this absence. Moreover, there is no typeface that supports all Unicode characters; therefore, the notdef glyph is unavoidable.

One frequent occurrence of the notdef glyph arises in the context of emoji interoperability between operating systems. As emoji support is typically introduced asynchronously across platforms, a character rendered correctly on one system may be unrecognized on another due to a lack of font or system-level integration. For instance, newer emojis are often first implemented on iOS before becoming available on Android, which may result in the display of a fallback glyph, commonly the notdef symbol, on devices that lack the corresponding graphical definition.

Representations

Common representations of the "notdef glyph".

The notdef glyph can vary graphically from one font to another. However, despite this variety, it is generally represented in a recognizable way to clearly signal the absence of a specific character. Common examples of frequently observed forms include:

• An outline of a square or rectangle, this basic geometric shape allows quick and clear identification of the notdef glyph;
• An outline of a square, rectangle, or diamond with a question mark in the center, visually emphasizes the uncertain or undefined nature of the missing character;
• An outline of a square or rectangle with a cross in the center, creating a distinctive notdef glyph with a universal connotation of error.

Etymology

The term "notdef glyph" is composed of two parts:

1. notdef: A contraction of "not defined", meaning "undefined" or "not definable". In the context of fonts, it refers to the absence of a graphical definition for the given character.
2. Glyph: From Ancient Greek, a graphical representation of a character or symbol in a font. A glyph is the basic unit used for text display.

See also

• Unicode
• Font
• Emoji
• Character encoding
• Typographic symbol