A crucial component for turning any temporal reasoning system into a real-world application that can be adopted by a wide base of users is given by its user interface. After analyzing and discussing the state of the art for the visualization of temporal intervals and relations, this paper proposes three new solutions to the problem of visualizing temporal intervals and their relations for querying databases containing several histories. The metaphors exploited in the proposed visual vocabularies are based on real-world, concrete objects, such as strips, springs, weights, and wires. We discuss the expressivity of the visual vocabularies with respect to the well-known Allen's Interval Algebra. A method for mapping queries composed by the visual vocabularies into SQL queries is then described and discussed. The proposed solutions were evaluated with two proper user studies: the first focused on determining which of the adopted metaphors are more frequently perceived and understood in a correct way and was based on a questionnaire; the second considered the two solutions which scored better in the first phase and studied them with a more thorough experiment, which was also based on user interfaces implementing the two proposals. The visual vocabulary which provided the best results has been adopted in a medical system for visual querying clinical temporal databases.
