An additional A–Z letter can be added as a sub arrangement of subordinate concepts. Generally, the letter matches the caption. If the caption has the same letter, then it is further subdivided by a number (ex: QW 138.5.E5 and QW 138.5.E8). The A–Z may remain the same even if the caption changes. The .5 number is a placeholder and is not used alone.
Note QW 138 is subdivided into an A–Z section (QW 138.5). The "Specific Organisms" use some, but not all, A–Z labels with several having the same letter (QW 138.5.E and QW 138.5.S).
| Class Number | Caption |
|---|---|
| QW Microbiology. Immunology Specific Bacteria |
|
| QW 138 | Enterobacteriaceae |
| QW 138.5 | Specific organisms, A-Z |
| QW 138.5.E5 | Enterobacter |
| QW 138.5.E8 | Escherichia |
| QW 138.5.K5 | Klebsiella |
| QW 138.5.P7 | Proteus |
| QW 138.5.S2 | Salmonella |
| QW 138.5.S3 | Serratia |
| QW 138.5.S4 | Shigella |
| QW 138.5.Y3 | Yersinia |