East Punjab (known simply as Punjab from 1950) was a state of India from 1947 until 1956, consisting of the parts of the Punjab Province of British India that went to India following the partition of the province between India and Pakistan by the Radcliffe Commission in 1947. The mostly Muslim western parts of the old Punjab became Pakistan's West Punjab, later renamed as Punjab Province, while the mostly Sikh and Hindu eastern parts went to India.
The princely states of the Punjab region (which had not been British possessions, so could not be partitioned by the British) all acceded to the new Dominion of India and were combined into the Patiala and East Punjab States Union (PEPSU).
The Constitution of India, which came into effect in 1950, renamed East Punjab as "Punjab".
In 1956, the PEPSU was merged into an expanded Punjab state. Later, with effect from 1 November 1966, there was yet another reorganization, this time on linguistic lines, when the state of Punjab as constituted in 1956 was divided into three: the mostly Hindi-speaking part became the present-day Indian state of Haryana and the mostly Punjabi-speaking part became the present-day Punjab,[1][2] while a new union territory (Chandigarh) was also created, to serve as a capital to both states. At the same time, some parts of the former territory of Patiala and East Punjab States Union, including Solan and Nalagarh, were transferred into the state of Himachal Pradesh.
Since it ceased to be the name of a state, "East Punjab" has been used in India to refer to the eastern part of the present Punjab state, while in Pakistan it means the eastern part of Pakistan's Punjab province, although Pakistanis also sometimes refer to the current Indian Punjab as "East Punjab".[3]