How did Shahjahanabad become known as Old Delhi?

Shahjahanabad, the capital city founded by the Mughal emperor Shah Jahan in 1639 and completed in 1648, is known today as Old Delhi.12

The transition in nomenclature is largely a result of the city's historical evolution and the subsequent development of the modern administrative center. While Shahjahanabad served as the seat of the Mughal Empire, the British later shifted the capital of India from Calcutta to Delhi in 1911. To accommodate this, they constructed a new administrative center, New Delhi, which was completed in 1931.2

As the metropolitan area expanded, the original walled city of Shahjahanabad became distinguished from the newer British-built capital, leading to its common designation as Old Delhi. Today, much of Old Delhi remains confined within the original walls built during Shah Jahan's reign, preserving historical landmarks such as the Red Fort and the Jama Masjid.21