PHILADELPHIA It has been used with great effect to challenge those in power and bring about social change, to further the civil rights movement, to end apartheid in South Africa, and to advance the cause of the United Farm Workers in California's San Joaquin Valley.

An exhibit opening Wednesday at the offices of the American Friends Service Committee (AFSC) titled "Boycott! The Art of Economic Activism" illustrates the power of boycotts in bringing about change around the world over the last 50 years.

The posters cover efforts as diverse as the 1955-56 Montgomery bus boycott, which launched the civil rights movement and made Rosa Parks and the Rev. Dr. Martin Luther King Jr. household names, to the boycott of Nestlé in the '70s and '80s for promoting infant formula in developing countries through false claims about dangers of breast feeding.

"We want observers to come in and look at the posters and begin to think about the purchases they are making," said Michael Merryman-Lotze, an AFSC official who is overseeing the exhibit in Philadelphia.

He said observers should come away thinking about how the products they buy "have an impact on their community, their country, and globally."

The exhibit, which has been displayed in Washington, Los Angeles, Indianapolis, and other cities, was produced in collaboration with Los Angeles' Center for the Study of Political Graphics. Many of the posters came directly from the center, while others are parts of individual collections, Merryman-Lotze said.

Each poster comes with the name of the artist, the year it was produced, and an explanation of the boycott it promoted.

One hand-drawn, black-and-white poster from 1955 takes aim at the Yellow Cab Co. in San Francisco. It promotes a boycott launched by the local NAACP demanding that black people be allowed to drive taxis in the city. The poster's text reads, "Say, 'We Won't Ride if Negroes Can't Drive.' "

Merryman-Lotze noted that almost all of the 52 posters in the exhibit are related to minority issues.

"We have campaigns that are focused on the LGBTQ community, campaigns from the African American community, and the immigrant communities," he said.

Economic boycotts are often effective because "they directly impact the source of power of the institution it is directed at," said Gwen Snyder, executive director of Philadelphia Jobs With Justice, a coalition of unions, community groups, and other parties that works for fair treatment of workers.

She said the exhibit "is a great resource for folks who aren't activists to draw from to understand the history of their movements."

Among the posters are several for the boycott promoted by the United Farm Workers against nonunion grape and lettuce growers in California in the 1960s and 1970s.

Two of those posters come with simple but powerful messages.

One shows an American Indian clutching bunches of grapes. Its text urges: "Boycott Grapes: Support the United Farm Workers Union."

Another shows women, men, and children in a field. Its text reads: "¡Sí, Se Puede! - It Can be Done: Boycott Grapes & Lettuce." 215-854-5717