Mr. and Mrs. Folger stored most of the material acquired for their future library in warehouses and safes in New York. Accordingly, individual items are associated with a "case number" indicating which packing crate they were in. Folger catalogers record the case number as "cs" followed immediately by the number (no space) as if it were an accession number, but it is important to know that these are not unique identifiers. Everything else in that case will have the same number.
There are three series of numbers:
- Case numbers: serial numbers from 1 through 2067 on typed lists, plus a few stragglers (highest number found is in the 2200s)
- Circle case numbers: serial numbers from 1 through 42 on typed lists, and up to 128 in boxed catalogs. Handwritten numbers have circles drawn around them, typewritten numbers have parentheses on either side.
- "S Lock" numbers (for "Safe Lock"?), numbered 0063, 0114, 0361, 0677, and 0900 in the boxed catalogs, and including 0822 in catalog records. No list has been found yet.
The four-digit accession numbers used by the library from October 20, 1930 through August 31, 1938, are often mistaken for case numbers (so sometimes have "cs" in front of the number in the catalog). They can be distinguished from case numbers because they run from 3001 through 7029, while case numbers end in the low 2000s.