Layout managers are used to control the placement of widgets in graphical user interfaces (GUIs). Constraint-based layout managers are more powerful than other ones. However, they are also more complex and their layouts are prone to problems that usually require direct editing of constraints. Today, designers commonly use GUI builders to specify GUIs. The complexities of traditional approaches to constraint-based layouts pose challenges for GUI builders.We evaluate a novel GUI builder, the Auckland Layout Editor (ALE), which addresses these challenges by enabling GUI designers to specify constraint-based layouts via direct manipulation using simple, mouse-based operations. These operations hide the complexity of the constraint-based layout model, while giving designers access to its benefits.In a user evaluation we compared ALE with two other mainstream layout builders, a grid-based and a constraint-based one. The time taken to create realistic sample layouts with our builder was significantly shorter, and most participants preferred ALE's approach. The evaluation demonstrates that good usability for authoring constraint-based layouts is possible.