# Z notation
The Z notation is a formal specification language for describing computing systems, based on typed set theory and first-order predicate logic, in which specifications are structured by units called schemas.[1](https://www.cs.ox.ac.uk/publications/publication3791-abstract.html) • [2](https://researchportal.lsbu.ac.uk/ws/portalfiles/portal/6315197/e0b10811_setss2014.pdf) It was developed by the Programming Research Group at Oxford University under Jean-Raymond Abrial around 1980, and the name comes from Zermelo-Fraenkel set theory.[3](https://onlinelibrary.wiley.com/doi/full/10.1002/spe.2634) Z is a specification notation rather than a full formal method: it describes what a system should do, not how to develop code from that description, and a Z specification is not executable.[4](https://www.researchgate.net/publication/2480325_Formal_Specification_and_Documentation_using_Z_A_Case_Study_Approach) • [5](https://etheses.whiterose.ac.uk/17776/1/thesis_acp12mus_rev.pdf)
| Key fact | Detail |
|---|---|
| What it is | A formal specification notation based on typed set theory and first-order predicate logic, used for discrete computer systems[2](https://researchportal.lsbu.ac.uk/ws/portalfiles/portal/6315197/e0b10811_setss2014.pdf) |
| Central construct | The schema, a named box combining declarations and predicates; schemas can be included in one another and combined with schema operators[1](https://www.cs.ox.ac.uk/publications/publication3791-abstract.html) • [2](https://researchportal.lsbu.ac.uk/ws/portalfiles/portal/6315197/e0b10811_setss2014.pdf) |
| Origin | Programming Research Group, Oxford University, under Jean-Raymond Abrial, around 1980; named for Zermelo-Fraenkel set theory[3](https://onlinelibrary.wiley.com/doi/full/10.1002/spe.2634) |
| Standard | ISO/IEC 13568:2002, covering syntax, type system, semantics, a mathematical toolkit, and LaTeX and e-mail mark-ups[6](https://www.iso.org/cms/%20render/live/en/sites/isoorg/contents/data/standard/02/15/21573.html) |
| Best-known industrial use | Specification of new modules of IBM's CICS transaction processing system from 1981; Queen's Award for Technological Achievement, April 1992[7](https://people.cs.nott.ac.uk/pszrq/files/SpecificationCaseStudies.pdf) |
| Status | A specification language, not executable; no code generator is generally agreed to be available, and industrial practice has largely moved to tool-centered methods such as B/Event-B, Alloy, and TLA+[3](https://onlinelibrary.wiley.com/doi/full/10.1002/spe.2634) • [8](https://codearchaeology.dev/languages/z-notation/) |
## Schemas and the mathematical toolkit
A Z specification has two layers: a mathematical language and a schema language.[9](https://eprints.soton.ac.uk/165419/1/ThesisM.pdf) The mathematical layer draws on set theory, relations, functions, and sequences, and the reference manual presents it as a "mathematical tool-kit" in which each symbol is given a formal definition in Z itself, an informal description, and a collection of algebraic laws.[10](https://www.informatik.uni-bremen.de/agbs/lehre/Books/Z/zrm.pdf) Reading a specification assumes a basic knowledge of naive set theory and predicate calculus.[10](https://www.informatik.uni-bremen.de/agbs/lehre/Books/Z/zrm.pdf)
The schema is the main structuring device. A schema consists of a collection of named objects specified by axioms, and Z provides notation for combining schemas, which allows large specifications to be built up in stages.[1](https://www.cs.ox.ac.uk/publications/publication3791-abstract.html) In the vertical form, declarations appear above a dividing line and predicates below; the same schema can be written horizontally in square brackets, and new schemas may be defined by combining old ones with the operations of the schema calculus.[10](https://www.informatik.uni-bremen.de/agbs/lehre/Books/Z/zrm.pdf) The schema language can describe the state of a system and the ways in which that state may change.[11](https://www.cs.cmu.edu/~15819/zedbook.pdf) A classic teaching example is the birthday book, which records people's birthdays and issues reminders on the appropriate day; its state schema declares a set of known names and a function from names to birthdays, with an invariant tying the set to the function's domain.[10](https://www.informatik.uni-bremen.de/agbs/lehre/Books/Z/zrm.pdf)
Schemas give specifications a modular structure: they can be named, included, combined, and reused, and the schema calculus is used to modularize a specification.[8](https://codearchaeology.dev/languages/z-notation/) • [10](https://www.informatik.uni-bremen.de/agbs/lehre/Books/Z/zrm.pdf) Composition mechanisms are rated well in comparative assessments, supported by well-defined theories of schemas and of promotion.[3](https://onlinelibrary.wiley.com/doi/full/10.1002/spe.2634)
## Refinement and development
Z includes data refinement, the process by which abstract specifications are developed into more concrete designs, and operation refinement for sequential programs.[10](https://www.informatik.uni-bremen.de/agbs/lehre/Books/Z/zrm.pdf) Its refinement theory is well developed, with technically complete forward and backward refinement proof rules.[3](https://onlinelibrary.wiley.com/doi/full/10.1002/spe.2634) Refinement cannot take development all the way down to the code level: as Bowen writes, Z permits a degree of data and operation refinement, yet reaching code eventually requires a leap that is usually made in an informal way.[3](https://onlinelibrary.wiley.com/doi/full/10.1002/spe.2634)
The [B-Method](https://www.edgepedia.org/b-method) grew out of this tradition. Its Abstract Machine Notation is described as a formal method based on Z, supported by an industrial-quality toolset, and the later Event-B method and its tools Rodin and ProB were shaped by practical experience with B, Z, and VDM.[12](https://www.sciencedirect.com/science/article/pii/095058499599366U) • [13](https://pure.au.dk/portal/en/publications/on-fitting-a-formal-method-into-practice)
## Industrial use: the CICS project
IBM (UK) Laboratories and the Oxford Programming Research Group ran a joint research project from 1981 to investigate whether formal specification techniques could scale to industrial software.[7](https://people.cs.nott.ac.uk/pszrq/files/SpecificationCaseStudies.pdf) The target was CICS (Customer Information Control System), IBM's main System/370 online transaction processing system, used for banking, airline reservations, and insurance, and by the early 1980s consisting of over 800,000 lines of code.[7](https://people.cs.nott.ac.uk/pszrq/files/SpecificationCaseStudies.pdf) After early case studies by Oxford researchers, part of the CICS program was restructured and rewritten with Z specifications for the new code; Z was used on approximately 14 percent of that new code, still amounting to several tens of thousands of lines.[7](https://people.cs.nott.ac.uk/pszrq/files/SpecificationCaseStudies.pdf) A 1988 IBM paper by C. J. Nix and B. P. Collins, both IBM-affiliated at the time, reported a potential for significantly reducing the volume of software quality defects at ultimately little additional development cost.[14](https://research.ibm.com/publications/use-of-software-engineering-including-the-z-notation-in-the-development-of-cics)
In April 1992, Oxford University Computing Laboratory and IBM (UK) Laboratories Ltd were joint winners of the Queen's Award for Technological Achievement for the use of the Z notation in the development of a transaction processing system.[7](https://people.cs.nott.ac.uk/pszrq/files/SpecificationCaseStudies.pdf) • [11](https://www.cs.cmu.edu/~15819/zedbook.pdf) The most widely publicized claims about the CICS/ESA version 3.1 release were less than half the normal number of customer-reported errors and a 9 percent saving in total development costs; a 1996 evaluation in the *Journal of Systems and Software* concluded that the case study was important and valuable but that the quantitative claims had not been substantiated.[15](https://www.sciencedirect.com/science/article/pii/S0164121296001227) A later project at IBM Hursley produced a formal description of a large part of the CICS Application Programming Interface, which could be published because the API is a public interface.[7](https://people.cs.nott.ac.uk/pszrq/files/SpecificationCaseStudies.pdf)
Z has also been used in safety- and security-related development. The Tokeneer ID Station project, developed for the US National Security Agency by Altran Praxis, formalized its specification in Z, refined it to a formal design, and developed the software in SPARK; after the project became available to the wider community in 2008, only five errors were found under intense scrutiny.[16](https://eceasst.org/index.php/eceasst/article/view/1885)
## Standardization and language design
The international standard ISO/IEC 13568:2002, *Information technology — Z formal specification notation — Syntax, type system and semantics*, covers the syntax, type system, and semantics of the notation, a toolkit of widely used mathematical operators, and LaTeX and e-mail mark-ups; it explicitly excludes any method of using Z, though an informative annex describes one widely used convention. The standard was last reviewed and confirmed in 2026 and remains current.[6](https://www.iso.org/cms/%20render/live/en/sites/isoorg/contents/data/standard/02/15/21573.html)
The standard grew out of J. M. Spivey's *The Z Notation: A Reference Manual* (2nd edition, Prentice-Hall, 1992), a primary teaching text at Oxford.[17](https://www.cs.ox.ac.uk/teaching/courses/2011-2012/intro2spec/) Spivey's manual deliberately omits overloading, because identifying common components when schemas are joined is hard to reconcile with two distinct variables sharing a name, and omits formal inference rules for deriving theorems about specifications, on the grounds that experience was insufficient to make the rules interact smoothly.[10](https://www.informatik.uni-bremen.de/agbs/lehre/Books/Z/zrm.pdf)
## Tool support and current standing
Tool support for Z is at least medium by comparative assessment. Written in Java and distributed as open source, the CZT (Community Z Tools) effort provides a foundation for creating formal-methods tooling for Z along with its dialects; it offers a parser, typechecker, and printer covering Z, Object-Z, and Circus, an animator that handles only a restricted subset of Z, and jEdit and [Eclipse](https://www.edgepedia.org/eclipse-software) plugins for specification editing.[18](https://czt.sourceforge.net/manual.html) The ProB animator and model checker, primarily a B-Method tool, supports Z specifications through its ProZ extension.[19](https://prob.cs.hhu.de/w/index.php?title=ProZ) • [3](https://onlinelibrary.wiley.com/doi/full/10.1002/spe.2634) Proof tools for Z are considered hard to build well, partly because each proof tool effectively defines its own semantics and because changes of notation are a barrier to understanding.[20](https://www.researchgate.net/publication/2644718_Approaches_to_proof_in_Z_-_or_-_Why_effective_proof_tool_support_for_Z_is_hard)
As a language, Z does not directly support the modeling of concurrency; concurrent systems can instead be addressed through extensions such as Object-Z combined with CSP, or through Circus.[3](https://onlinelibrary.wiley.com/doi/full/10.1002/spe.2634) The notation itself has changed little since the 2002 ISO standard, the dedicated Z User Meetings have been absorbed into the broader ABZ conference, and industrial practice has largely moved to tool-centered methods such as B/Event-B, Alloy, and TLA+.[8](https://codearchaeology.dev/languages/z-notation/) Z remains in teaching and research use, including recent formalization work such as a Z specification of the W3C Draft Core SHACL Semantics.[21](https://arxiv.org/abs/1511.00384)
## References
1. [Mike Spivey (1984). Towards a Formal Semantics for the Z Notation. Oxford University Computing Laboratory, PRG-41.](https://www.cs.ox.ac.uk/publications/publication3791-abstract.html)
2. [The Z Notation (Sets Seminar notes), London South Bank University Research Portal.](https://researchportal.lsbu.ac.uk/ws/portalfiles/portal/6315197/e0b10811_setss2014.pdf)
3. [Mashkoor (2018). Evaluating the suitability of state-based formal methods for industrial deployment. Software: Practice and Experience.](https://onlinelibrary.wiley.com/doi/full/10.1002/spe.2634)
4. [Formal Specification and Documentation using Z: A Case Study Approach.](https://www.researchgate.net/publication/2480325_Formal_Specification_and_Documentation_using_Z_A_Case_Study_Approach)
5. [Support for Model Checking Z Specifications (thesis), White Rose eTheses.](https://etheses.whiterose.ac.uk/17776/1/thesis_acp12mus_rev.pdf)
6. [ISO/IEC 13568:2002 — Information technology — Z formal specification notation — Syntax, type system and semantics.](https://www.iso.org/cms/%20render/live/en/sites/isoorg/contents/data/standard/02/15/21573.html)
7. [Steve King (1992). The use of Z in the restructure of IBM CICS, in I. Hayes (ed.), Specification Case Studies.](https://people.cs.nott.ac.uk/pszrq/files/SpecificationCaseStudies.pdf)
8. [Z notation, CodeArchaeology.](https://codearchaeology.dev/languages/z-notation/)
9. [Thesis on Z specifications, University of Southampton Research Repository.](https://eprints.soton.ac.uk/165419/1/ThesisM.pdf)
10. [J. M. Spivey. The Z Notation: A Reference Manual, 2nd edition.](https://www.informatik.uni-bremen.de/agbs/lehre/Books/Z/zrm.pdf)
11. [The Z notation (course text), Carnegie Mellon University.](https://www.cs.cmu.edu/~15819/zedbook.pdf)
12. [Formal development in B abstract machine notation. Information and Software Technology.](https://www.sciencedirect.com/science/article/pii/095058499599366U)
13. [Gmehlich et al. On Fitting a Formal Method into Practice. Aarhus University.](https://pure.au.dk/portal/en/publications/on-fitting-a-formal-method-into-practice)
14. [C. J. Nix and B. P. Collins (1988). Use of software engineering, including the Z notation, in the development of CICS. IBM Research.](https://research.ibm.com/publications/use-of-software-engineering-including-the-z-notation-in-the-development-of-cics)
15. [Evaluating the effectiveness of Z: The claims made about CICS and where we go from here. Journal of Systems and Software 35(3), 1996.](https://www.sciencedirect.com/science/article/pii/S0164121296001227)
16. [Janet Barnes (2011). Experiences in the Industrial use of Formal Methods. ECEASST vol. 46.](https://eceasst.org/index.php/eceasst/article/view/1885)
17. [Introduction to Specification course, Oxford University Department of Computer Science, 2011-2012.](https://www.cs.ox.ac.uk/teaching/courses/2011-2012/intro2spec/)
18. [Community Z Tools (CZT) manual.](https://czt.sourceforge.net/manual.html)
19. [ProZ — ProB Documentation.](https://prob.cs.hhu.de/w/index.php?title=ProZ)
20. [Approaches to proof in Z — or — Why effective proof tool support for Z is hard.](https://www.researchgate.net/publication/2644718_Approaches_to_proof_in_Z_-_or_-_Why_effective_proof_tool_support_for_Z_is_hard)
21. [Z Specification for the W3C Editor's Draft Core SHACL Semantics. arXiv:1511.00384.](https://arxiv.org/abs/1511.00384)
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*Topic: Encyclopedia › Physical world and mathematics › Mathematics and statistics › Logic and discrete mathematics › Formal logic and foundations › Logical calculi and logical syntax › Lambda calculus and type theory › Dependent type theory and Martin-Löf type theory*
*Initially written Sep 17, 2026 · Reviewed: Oct 15, 2026 · Edited: Oct 15, 2026 · Last review: Oct 15, 2026*
*Copyright 2026 EdgeChat AI, a subsidiary of Biostate AI.*
License: Edgepedia Community License 1.0, https://www.edgepedia.org/edgepedia/license