This chapter comprises three notes from unpublished typescripts on the relationship between separability and the linearity of Engel curves, which have been put together into one paper with three sections. They were apparently all written about the same time, and one of them is dated March 1971. Engel curves are curves showing the relationship between income level and spending on the consumption of some good, at a given price. Preferences with linear Engel curves are a natural generalization of homotheticity, which yields straight-line Engel curves passing through the origin. The subject has appeared tangentially in some of the budgeting papers in Part I of the book, and reappears in the discussion of representative consumers and firms in Part II.
Keywords: budgeting, consumption, Engel curves, homotheticity, linear Engel curves, linearity, preferences, separability, spending