The second line is a tradition in parades organized by Social Aid and Pleasure Clubs (SAPCs) with brass band parades in New Orleans, Louisiana, United States. The "main line" or "first line" is the main section of the parade, or the members of the SAPC with the parading permit as well as the brass band. The second line consists of people who follow the band to enjoy the music, dance, and engage in community. The second line's style of traditional dance, in which participants dance and walk along with the SAPCs in a free-form style with parasols and handkerchiefs, is called "second-lining". It is one of the most foundationally Black American–retentive cultures in the United States. It has been called "the quintessential New Orleans art form – a jazz funeral without a body". Another significant difference from jazz funerals is that second line parades lack the slow hymns and dirges played at funerals (although some organizations may have the band play a solemn selection toward the start of the parade in memory of members who died since their last parade).

The second line has its origins in traditional West African circle or ring dances. The second line tradition was brought to New Orleans by enslaved Africans, where it became a ritual for African Americans, especially in various processions, including funerals. Some scholars believe that the West African ring featured children drumming with adults dancing that in turn, forced the ring to straighten into a line. Others note the similarity of the steps – exaggerated, stylistic strutting – to dances performed in Congo Square by the enslaved given the day off on Sundays. These dances were officially banned for a time because they were deemed threatening to the white inhabitants of the city, and their resurgence in the second line culture suggests a similar celebration of individual freedom. The second line tradition is a blending of African ritual dancing with European-styled military bands.

The movement of street dancers in the second line gathered force in the late 19th century as it joined the linear procession of military bands, rooted in French and Spanish colonial traditions. By the time jazz flowered, the funerals showed the coming together of the ring and the line, the African ancestral circle dancing and the linear progression of the brass bands’ marching music.

An article from New Orleans states that the history of second line traditions in New Orleans' Black neighborhoods began during the era of enslavement. Enslaved and free Black people began second lines as neighborhood celebrations. "The neighborhood organizations offered social aid to freed slaves, such as loans and insurance, and used the second-lines as a form of advertising. Second lines were also used to honor members who died in their community, which launched the idea of second lines at funerals. Often, Mardi Gras Indians were and still are included in the procession."

African and African-American traditions continued throughout the "Code Noir" and Jim Crow eras in New Orleans. African-Americans formed Benevolent Societies and "Social, Aid & Pleasure Clubs" because white insurance companies often refused to cover free people of color and/or the formerly enslaved. SAPCs assisted members through illness and supported families with burial costs for deceased members. This is a carry-over from African traditions that believed in celebrating the member's spirit leaving the body to return to the ancestors and God. This led to what became known as a "jazz funeral", with the SAPC members marching in a dirge with a brass band before the deceased body being "cut loose" and a celebratory parade begins. The same club exercised their social aspect with a colorful, annual, public second line parade through their home community.

A second line snare drummer commonly follows the brass band, playing off the marching beat with improvised polyrhythmic figures that can inspire second line dancers or, if the band is improvising, the band itself. Second line drumming styles became a feature of early jazz drumming and the New Orleans rhythm and blues of the 1950s. The Rebirth Brass Band and the Dirty Dozen Brass Band feature traditional second line drumming styles. Drummers such as Billy Higgins and Idris Muhammad adapted second line drum figures to modern jazz.

Second line parades are part of the cultural heritage of New Orleans.

The locally best known second line parades are held by clubs and benevolent organizations. Some have long histories; the oldest such organization still holding regular parades is the Young Men Olympian Junior Benevolent Association, founded in 1884. During the "second line season", lasting most of the year with breaks for holidays (including Mardi Gras) and the hottest part of summer, there are second line parades most Sundays. Longer parades often make stops, commonly at bars, where refreshments have been arranged for members and those following the parade for fun can purchase something. There are often vendors selling soft drinks, beer, and street food, including barbecue and yaka mein.