Rawalpindi, colloquially known as Pindi, is the fourth-most populous city in Pakistan, twinned with the capital city of Islamabad and serving as the principal commercial and industrial hub of northern Punjab. It is also the third-largest city in the province of Punjab and also ranks as the world's third-largest Punjabi-speaking metropolis. Located along the Soan River in north-western Punjab, Rawalpindi lies adjacent to Pakistan's capital, Islamabad, and the two are jointly known as the "twin cities".