Stephen Lack*

*Corresponding author for this work

Research output: Contribution to journalArticlepeer-review

41 Citations (Scopus)


Categorical orthodoxy has it that collections of ordinary mathematical structures such as groups, rings, or spaces, form categories (such as the category of groups); collections of 1-dimensional categorical structures, such as categories, monoidal categories, or categories with finite limits, form 2-categories; and collections of 2-dimensional categorical structures, such as 2-categories or bicategories, form 3-categories.We describe a useful way in which to regard bicategories as objects of a 2-category. This is a bit surprising both for technical and for conceptual reasons. The 2-cells of this 2-category are the crucial new ingredient; they are the icons of the title. These can be thought of as "the oplax natural transformations whose components are identities", but we shall also give a more elementary description. We describe some properties of these icons, and give applications to monoidal categories, to 2-nerves of bicategories, to 2-dimensional Lawvere theories, and to bundles of bicategories.

Original languageEnglish
Pages (from-to)289-307
Number of pages19
JournalApplied Categorical Structures
Issue number3
Publication statusPublished - Jun 2010
Externally publishedYes


  • 2-category
  • Bicategory
  • Icon
  • Oplax natural transformation
  • Pseudonatural transformation


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