Null sign

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Sign representing zero or empty set

title: "Null sign" type: doc version: 1 created: 2026-02-28 author: "Wikipedia contributors" status: active scope: public tags: ["mathematical-symbols"] description: "Sign representing zero or empty set" topic_path: "general/mathematical-symbols" source: "https://en.wikipedia.org/wiki/Null_sign" license: "CC BY-SA 4.0" wikipedia_page_id: 0 wikipedia_revision_id: 0

::summary Sign representing zero or empty set ::

::figure[src="https://upload.wikimedia.org/wikipedia/commons/0/0f/Atheism_symbol_syreeni.svg" caption="A null sign"] ::

The null sign (∅) is a symbol often used in mathematics for denoting the empty set. The same letter in linguistics represents zero, the lack of an element. It is commonly used in phonology, morphology, and syntax.

Encodings

The symbol ∅ is available at Unicode point U+2205. It can be coded in HTML as and as or as . It can be coded in LaTeX as .

Similar letters

Similar letters and symbols include the following:

Use in mathematics

In mathematics, the null sign (∅) denotes the empty set. Note that a null set is not necessarily an empty set. Common notations for the empty set include "{}", "∅", and "\emptyset". The latter two symbols were introduced by the Bourbaki group (specifically André Weil) in 1939, inspired by the letter Ø in the Danish and Norwegian alphabets (and not related in any way to the Greek letter Φ).

Empty sets are used in set operations. For example: ::quote

A={2,3,5,7,11}

B={4,6,8,9}

A \cap B = ? ::

There are no common elements in the solution; so it should be denoted as: ::quote

A \cap B = \varnothing or A \cap B = { } ::

Use in linguistics

In linguistics, the null sign is used to indicate the absence of an element, such as a phoneme or morpheme.

Morphology

The English language was a fusional language, this means the language makes use of inflectional changes to convey multiple grammatical meanings. Although the inflectional complexity of English has been largely reduced in the course of development, the inflectional endings can be seen in earlier forms of English, such as Early Modern English (abbreviated as EModE).

The verb endings of EModE were summarised in the table below by Roger Lass:

::data[format=table title="Verb Endings of EModE"]

PresentPast
First person singular-∅
Second person singular-st
Third person singular-th, -s
::

Use in photography

In photography the null sign, Ø, found on camera lenses denotes the filter thread diameter, measured in millimeters. This marking indicates the size of screw-in filters that can be attached to the front of the lens. This diameter is separate from any optical specification of the lens, as it is a standardized measurement for photographers to select a compatible filter. Common filter thread sizes include 52mm, 72mm, 77mm, and 82mm. If a photographer owns multiple lenses with different filter thread diameters, stepping rings can be utilized to adapt larger filters to smaller lens threads, eliminating the need to purchase duplicate filters in various sizes.

References

References

  1. [https://www.unicode.org/charts/PDF/U2200.pdf Unicode Standard 5.2]
  2. (January 21, 2007). "Some Common Mathematical Symbols and Abbreviations (with History)".
  3. (2012). "Introduction to Mathematical Structures and Proofs". Springer.
  4. "Earliest Uses of Symbols of Set Theory and Logic".
  5. (2013). "Phonetic Symbol Guide". University of Chicago Press.
  6. Lass, R. (1999). Phonology and Morphology. In R. Lass (Ed.), ''The Cambridge History of the English Language: 1476-1776'' (Vol. 3, pp. 137-180). Cambridge, UK: Cambridge University Press.

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