This Practice Note, together with Introduction to business finance and accounting—financial accounting and external performance, aims to help you understand your organisation through its financial language and methods. This Practice Note covers:•the difference between fixed and variable costs•how investments are appraised by the business•budgetingSee Practice Note: Introduction to business finance and accounting—financial accounting and external performance for guidance on financial accounting—the core financial statements, how to assess financial health, how markets value companies, and working capital.See also Checklists: How financially literate are you and Improving your financial literacy, which offer a series of questions and practical actions to help you assess and improve your financial literacy.Cost behaviourNot all business costs behave the same way:•some costs are fixed, meaning they don’t change with volume, eg rent•some are variable, meaning they change directly with volume, eg the materials used to make each productThis distinction matters because it determines how profit changes as volume