Aspects of the subject technology relate to systems and methods for representing complex constraints on widgets for a user interface, using a small set of common rules allowing developers to represent arbitrary one-to-one relationships between widgets. A layout tool may be provided that, in operation, displays a layout area to a developer. The layout tool may include a user interface (UI) inference engine that, when one or more widgets are placed and/or moved within the layout area, generates inferred constraints for the one or more widgets. These inferred constraints may be generated, with or without further input from the developer, based on the relative positions of the one or more widgets in the layout area. An infer-constraints button may be provided that, when selected following placement of widgets in the layout area, causes generation of inferred constraints for each of the widgets.