Town of Otowa
Otowa is the name of the town where the story of ef takes place. A major earthquake and fire had completely destroyed the original town when Yuu Himura was a young child. The earthquake killed Himura's sister Akane Himura, and it also killed Akira Amamiya's sister as well.
After the earthquake, a second town (also named Otowa) was built in Australia that was designed to be a duplicate of the original Otowa. Some of the displaced population from Otowa, Japan then moved to Otowa, Australia.
In the ending of Melodies, it is revealed that Yuu Himura (who is now an architect) had helped to design much of the second town.
Summer and Winter[]
In Memories, the events shift back and forth between summer and winter. For example, Hiro Hirono's arc takes place in winter, while Chihiro Shindou's arc takes place in summer. Normally this would indicate a time shift between events. But in this case the events are actually happening simultaneously. This is because the events are happening in two identical towns both named Otowa. The second city is located in Australia (in the southern hemisphere) and so the seasons are reversed. This is not explained in Memories, and it is not until Melodies that the situation is fully explained to the viewer.