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Nappa leather is a leather, typically dyed, made from kid-, cow-, calf-, lamb- or other skin by tanning. It is noted for its soft temper. It is a generic term in the leather field and has no distinct test for characterization. Because of this ambiguity, the term is  to imply that a leather has a soft hand. Among other uses, Nappa leather is often used in leather products such as furniture, clothing, handbags, and shoes. As the term is used today, Nappa Leather may be either natural grain, or more likely, corrected grain.
Nappa tannage in the "strict sense", but a term no longer used in this framework, refers to a tannage consisting of alum salts with vegetable tanning agents.
Napa leather is defined in Merriam-Webster's dictionary.
B Ellis (1921), Gloves & Glove Trade, page 58,
Napa gloves are made from tawed leathers and tanned leathers.
Emanuel Manasse - obituary article (1899) and patent (1875), retrieved 11/08/2010 from Ancestry.com