Canons of construction are the interpretative principles and presumptions courts use to read legal texts-principally statutes, but also contracts, deeds and wills. The expression is descriptive, not defined in legislation; the canons arise from case law and long‑standing maxims, applied alongside the Interpretation Act 1978 (E&W), the Interpretation and Legislative Reform (Scotland) Act 2010, the Interpretation Act (Northern Ireland) 1954 and Ireland’s Interpretation Act 2005.Common canons include: the purposive/mischief approach (Heydon’s Case) alongside ordinary meaning; ejusdem generis; noscitur a sociis; expressio unius est exclusio alterius; contra proferentem; presumptions against retrospectivity, against ousting the courts’ jurisdiction and against interfering with fundamental rights (principle of legality). They are non‑hierarchical, context‑sensitive and may point in different directions; clear text and overall statutory or contractual context prevail.In practice, advocates deploy canons to resolve ambiguity, choose between rival readings, address silence, and support or resist implied terms. They may justify limited use of extrinsic materials (e.g. Pepper v Hart and Explanatory Notes) in the UK.Usage is broadly consistent across England & Wales, Scotland and Northern Ireland, influenced by section 3 Human Rights Act 1998. In Ireland, similar canons apply, shaped by the Constitution, the presumption of constitutionality and the “double construction” rule. Canons aid, but do not replace, textual and contextual analysis.