Primary: these are original materials which have not been filtered through interpretation, condensation, or evaluation by a second party; for example journal articles, reports, patents, theses, diaries, letters, photographs, poems. Secondary: information about primary, or original, information which has been modified, selected, or rearranged for a specific purpose or audience. It is not always easy to discern the difference between primary and secondary sources. Examples include biographies, histories, monographs, review articles, textbooks, and any index or bibliography used to locate primary sources. Tertiary: these consist of information which is a distillation and collection of primary and secondary sources. Twice removed from the original, they include encyclopedias, fact books and almanacs, guides and handbooks. Some secondary sources such as indexing and abstracting tools can also be considered tertiary sources. See Types of Information Sources for a more detailed explanation.