Couples living in long-distance relationships (LDRs) may lack ways to keep emotionally connected. Previous research has presented a wealth of systems and user studies that offer insights of individual systems and their user interface designs. These studies have revealed a multitude of design attributes of the relatedness strategies of LDRs and the user interfaces used in computer-mediated communication (CMC) systems for LDRs. In this paper, we synthesise the multitude of different design attributes, and present a design framework that addresses the five main areas of LDR systems: users (the remote couple), the LDR itself, the used technology, the design of the device, interaction, nature of messages and supported connectedness strategies, and the context of the use. We validate the framework by analysing and presenting a set of six existing systems and prototypes in light of this framework, and show how they take into account the central design attributes. As a conclusion, we propose that this framework can be used to assist in designing and evaluating the user interfaces of CMC systems for emotional communication to support LDRs.