.cs was for several years the country code top-level domain (ccTLD) for Czechoslovakia. However, the country split into the Czech Republic and Slovakia in 1993, and the two new countries were soon assigned their own ccTLDs: .cz and .sk respectively. The use of .cs was gradually phased out, and the ccTLD was deleted some time around January 1995.
Until the deletion of .yu in 2010, .cs was the most heavily used top-level domain ever to be deleted. Statistics from the RIPE Network Coordination Centre show that even in June 1994, after much of the conversion to .cz and .sk had been done, .cs still had over 2,300 hosts. By comparison, other deleted TLDs ( .nato and .zr) may never have reached 10 hosts.
In July 2003, CS became the ISO 3166-1 code for Serbia and Montenegro (Srbija i Crna Gora in Serbian), and remained so until 2006, when the country split and the codes for domains .rs and .me were created. However, Serbia and Montenegro did not use .cs as its ccTLD, but continued instead to use the Yugoslavian ccTLD .yu.