The original application of the term is to music of the 1880s Tin Pan Alley period in the United States. Although popular music sometimes is known as "pop music", the two terms are not interchangeable. Popular music is a generic term for a wide variety of genres of music that appeal to the tastes of a large segment of the population, whereas pop music usually refers to a specific musical genre "within" popular music. Popular music songs and pieces typically have easily singable melodies. The song structure of popular music commonly involves repetition of sections, with the verse and chorus or refrain repeating throughout the song and the bridge providing a contrasting and transitional section within a piece.