 The National Archives Catalog is our online portal to the records held at the National Archives and information about those records. It's our main way of describing our holdings so that you can learn about what we have. The Catalog is also how we provide access to electronic records and digitize versions of our holdings. So it's a great resource even if you're unable to visit a facility in person. An important thing to understand is that only a small percentage of our holdings are available online. Not every page of records we have at the National Archives is in our Catalog. Our Catalog also searches across multiple National Archives resources at once. So performing a search will often retrieve digital files of records, record descriptions, but also web pages, blog posts, and Presidential Library websites. The other exciting feature of our Catalog is that it allows you to participate in enhancing historical records through tagging, transcription, and commenting. I first want to explain how records are organized or arranged at the National Archives. We call this record hierarchy. Understanding the hierarchy of the types of descriptions you see in the Catalog will help you get a feel for how records are organized and will also help you search for and identify them when you see them in the Catalog. And when you find a document of interest, it can help you find more like it. I like to think of the National Archives as the nation's filing cabinet, which can be a helpful way to visualize how we organize records. At the very top of the hierarchy, we have record groups. Each record group comprises the records of a major government entity, usually a bureau or an independent agency that created or maintained these records. For example, record group 255 is records of the National Aeronautics and Space Administration. It might be helpful to think of a record group as an entire file cabinet. It has many different drawers and folders and items within those drawers, but the records are united by the federal agency that created or maintained those records. This is the largest bucket and a very high-level grouping of records. Another thing to note is that records within one record group are not always found in the same location. The National Archives is not just in Washington, D.C. We have various regional archives and presidential libraries across the country, and records within a record group may be held at one or more of those locations. The next level in our hierarchy is series, which further organizes records of a government agency. You can think of a series as a drawer within a file cabinet. Records that have the same function or were created by the same activity are in a series together. An example of a series is mission photographs taken during the Space Shuttle Program 1981 to 2011, which is part of record group 255, records of NASA. A series can be one box or it could be thousands of boxes. You can find the extent of a series by looking at its catalog description. 95% of our records are described at the series level. This means you can find out basic information about the records, including how big it is, from a few linear inches to thousands of linear feet from its description in the catalog. You can also find out where the records are located. A file unit is the next smaller level in our hierarchy. You can think of file units as folders within a drawer. They're not always just one folder. Sometimes the records may need to be housed in multiple folders. A file unit could even be a bound volume. An example of file unit is STS-47, which was the 50th Space Shuttle mission of the program. An item is the lowest level of description we have. It represents an individual archival record and all the pages that make it up. Items can be letters, memos, reports, or photographs, and they can also be sound recordings or films. For example, this photograph in our catalog of astronaut Mage Emerson is one item within the file unit STS-47 from the series Mission Photographs Taken During the Space Shuttle Program, which is part of Record Group 255 Records of NASA. You can always see this record hierarchy here in our catalog. When you're looking at a description, you'll notice the hierarchy in the upper left-hand corner. This shows you where the item lives within the hierarchy I just discussed. Each level is a link, so clicking on each of these links will allow you to back up to the Record Group or series or file unit where you can find more information. For instance, if you want to find more records like the one you're looking at, back up to the file unit or series where you can find similar records. The holdings of the National Archives are growing all the time and we're adding more digital objects to our catalog each week, so you're likely to find something new every time you search.