Which type of literature is considered twice removed from the original source?

Tertiary literature is considered twice removed from the original source because it compiles and summarizes information from primary and secondary literature. Primary literature refers to original research, experiments, or firsthand accounts. Secondary literature interprets and analyzes primary sources, often synthesizing multiple studies or viewpoints. Tertiary literature, such as encyclopedias, textbooks, and databases, takes this information a step further by providing a broad overview or summary of knowledge, making it more accessible but also more distant from the detailed original research. This layering of information makes tertiary literature a valuable resource for finding consolidated data and context, although it may lack the depth and specificity found in primary and secondary sources.