Soyuz 4 and 5 docked on January 16, the first time two manned spacecraft had docked (Apollo 9 would do the same in March of the same year). The two craft possessed only a primitive probe (Soyuz 4) and drogue (Soyuz 5) docking assembly. A connecting tunnel for the docking mechanism had not yet been developed, which prevented a simple internal transfer between the craft. This required the two transferring cosmonauts to spacewalk from one vehicle to the other.
Wikipedia
by Webb » Mon Jan 16, 2012 10:31 pm 
