Taking inspiration from the work of Douglass North, much institutional research attempts a distinction between ‘formal’ and ‘informal’ institutions. North often associated ‘formal institutions’ with rules enforced through a legal system. It is suggested here that this lead should be followed and refined. In which case ‘legal system’ and ‘law’ require definitions. An alternative claim, that ‘formal’ basically means ‘written down’, is arguably less useful. Stressing the importance of clear definitions in this area, this paper considers a case where slight modifications yield strikingly different results. Some options concerning the meanings of ‘culture’ and their relation to institutions are briefly noted. Changes in, and interactions between, ‘formal’ and ‘informal’ institutions are considered, with illustrative examples. Contrary to some authors, informal institutions can sometimes change rapidly, in some cases in response to state legislation.