This is a different meaning from that of the word retablo in Spanish, and is equivalent to reredos in English or retable in French: a painting, sculpture or combination of the two, rising behind the altar of a church. The Latin etymology of this Spanish word means "board behind".
Spanish retablos of the Late Middle Ages and Renaissance grew extremely large and elaborate, typically using carved and gilded wood, and rising as high as 40 feet or more. The tradition of making them was taken to the new Spanish Empire in America. There, by the late 18th century at least, the word became used for much smaller popular religious paintings, both conventional devotional images and ex-votos (paintings giving thanks for protection through a specific episode).
Historical usage 
Aside from being found behind the altar, "similar ornamental structures are built and carved over facades and doorways" (Fernandez 23), called overdoors. Labeling ex-votos as retablos can be traced back to the early 18th century from a man named Robert Montenegro. He published a collection of votive paintings that was dated 1781. He was thanking Nuestra Señora de Dolores de Xaltocan for renewing the people’s health after a severe illness. On the bottom of the retablo left, he inscribed a message that read "en cuia memoria dedica a su Magestad este Retablo" (in whose memory he dedicates to her Majesty this retablo) (Durand, 5).
The oil painted retablos generated the need for "small retablo factories" to be established in order to "reproduce the same images" which were then "sold to devout believers who displayed them in home altars to honor their patron Saints." Not only were the retablos purchased by those wanting to show devotion to their patron saints, they were also given when the saints were there for their devotees in hard times. When one wanted to express gratitude, they could give retablos that described "the miraculous deed of a saint to whom the petitioner turned to in a time of need" (Correa 113). People call upon these saints for aid with rain, harvests or other outcomes. For example, if a farmer needs rain for his crops he might pray for rain. After the rain comes, a retablo might be created to give thanks to San Ysidro Labrador, the patron saint of farmers. He is "venerated for good weather, agricultural issues and prosperous crops" (Eddy, 1). That figure would most likely be kept in the farmer's house. Each time rain was needed in the future, that farmer might pray in front of that retablo (Eddy, 1). These traditions are by no means exclusive to Latin America, but are found in all Roman Catholic countries, as well as in classical paganism and many other religions. By the 19th century, the process of lithography became very popular for devotional retablos, replacing earlier printmaking methods.
Retablos are devotional or votive paintings, often on rectangular sheets of tin that illustrate holy images such as Christ, the Virgin Mother, or one of the hundreds of saints. Ex-votos ("from a vow") are different from retablos in that they often depict the story that led to their commission. These stories recall dangerous or threatening events that actually occurred, and which the person survived, thanks to the intercession of a sacred person - God, Mary or a saint. They are made as a way of thanking the sacred person for protection in precarious situations, such as surviving an illness or earthquake. On a strict definition, this class of ex-votos should display the protected humans in the dangerous situation, and the sacred person who protected them, usually with an inscribed explanation of the events, with the date and location. Both devotional and especially ex-voto retablos may be deposited at a shrine as a votive offering, or alternatively kept at home.
The way a retablo and ex-voto look is entirely up to the person designing it, so long as it contains the necessary basic elements. The most important part of the retablo is the representation of the miraculous event. That is why most artists try to use bright, vibrant colors to portray the supremacy of the event. An ex-voto is, more often than not, reduced to a smaller size. It’s usually about half the size of the original, while still maintaining its rectangular shape. The largest size was about 280 square inches. The smallest was about 18 square inches. Some of the most common sizes include 140 square inches and 70 square inches (Durand, 9).
Retablos are important to Mexican folk religion because they are a physical representation of holy images such as Christ, the Virgin Mother, or one of the many thousands of saints. They come from the need humans have to interact on a personal level with divine spirits. Retablos are evidentiary support for communication between the divine and humans (Wroth, 2).
This tradition of retablos was also brought into New Mexico and southern Colorado by the Franciscan monks. Due to the remoteness of this frontier and lack of metals, retablos were made of wood. These crude retablos were coated with a gesso made with gypsum and rabbit skin glue. Pigments also were made locally from natural materials, colored earths, plant extracts, cochineal bugs, and lamp black.
These traditional retablos and other indigenous religious art were removed by Bishop Lamy throughout New Mexico after the conquest of these territories by the US Army. In 1924 the Spanish Colonial Arts Society was formed, and since then these unique traditions of New Mexico have been preserved.
Contemporary Usage 
During the 1940s, a resurgence of the art began in Peru, due in large part to the research and philanthropy of Alicia Bustamante, a member of the Peruvian indigenista movement, who encouraged an artist named Joaquin Lopez Antay to save and revitalize the retablo art form. She enlisted him "to make retablos that included themes of everyday life – harvests, markets, and fiestas. A distinction emerged between the retablo for ritual and religious purposes and the retablo as decoration." Many others in the area, including Nicario Jimenez Quispe, continued along the same path as Antay, creating a popular art that was displayed during annual branding ceremonies of cattle, sheep, and llamas. These retablos have achieved status as important heirlooms passed on within families that symbolize protection, fertility and healing. They are also sold as art.
See also 
- Correa, Phyllis M. Otomi Rituals and Celebrations: Crosses, Ancestors, and Resurrection. The Journal of American Folklore 113 (2000): 436–450.
- Durand, Jorge and Massey, Douglas S. (1995). Miracles on the Border: Retablos of Mexican Migrants to the United States. Tucson; London: The University of Arizona Press.
- Orozco Palma, Miguel. The Santa Heduwiges Retablo in my family. Nuevo Casas Grandes, Chihuahua, Mexico.
- Eddy, James. Mexican Retablos 19th Century Devotional Art. San Francisco, California.
- Fernandez, Justino. An Aesthetic of Mexican Art: Ancient and Modern. The Journal of Aesthetics and Art Criticism 23 (1964): 21–28.
- Lange, Yvonne. Lithography, an Agent of Technological Change in Religious Folk Art: A Thesis. Western Folklore 33 (1974): 51–64.
- Wroth, William. (Jul–Sep 1984). Christian Images in Hispanic New Mexico. The Journal of American Folklore 97 (385): 361–363.