Legends are important for understanding maps. Legend design is a part of map design and forms an important topic of cartographic research. Most research on legend design concentrates on the development of techniques rather than the development of basic principles. This study is devoted to the latter topic. Particularly, attention is paid to the development of spacing and alignment rules for effective design of legends shown on screens (computer monitors and tablet screens). Based upon Gestalt laws and Bumstead's rules, a set of spacing and alignment rules is developed. Experimental evaluations are conducted, and the results indicate that a legend designed with proper consideration of the spacing and alignment rules is much more effective and efficient than ordinary legends.