| Q - Science | |
| QA - Mathematics | |
| QB - Astronomy | |
| QC - Physics | |
| QD - Chemistry | |
| QE - Geology | |
| QH - Natural history, Biology | |
| QK - Botany | |
| QL - Zoology | |
| QM - Human anatomy | |
| QP - Physiology | |
| QR - Microbiology | |
| R - Medicine | |
| RA - Public aspects of medicine | |
| RB - Pathology | |
| RC - Internal medicine | |
| RD - Surgery | |
| RE - Ophthalmology | |
| RF - Otorhinolorayngology | |
| RG - Gynecology and obstetrics | |
| RJ - Pediatrics | |
| RK - Dentistry | |
| RL - Dermatology | |
| RM - Therapeutics, Pharmacology | |
| RS - Pharmacy | |
| RT - Nursing | |
| RV - Botanic, Thomsonian, and eclectic medicine | |
| RX - Homeopathy | |
| RZ - Other systems of medicine | |
| S - Agriculture | |
| SB - Plant culture | |
| SD - Forestry | |
| SF - Animal culture | |
| SH - Aquaculture, Fisheries, Angling | |
| SK - Hunting sports | |
| T - Technology | |
| TA - Engineering (General). Civil engineering | |
| TC - Hydraulic engineering. Ocean engineering | |
| TD - Environmental technology. Sanitary engineering | |
| TE - Highway engineering. Roads and pavements | |
| TF - Railroad engineering and operation | |
| TG - Bridge engineering | |
| TH - Building construction | |
| TJ - Mechanical engineering and machinery | |
| TK - Electrical engineering. Electronics. Nuclear engineering | |
| TL - Motor vehicles. Aeronautics. Astronautics | |
| TN - Mining engineering. Metallurgy | |
| TP - Chemical technology | |
| TR - Photography | |
| TS - Manufactures | |
| TT - Handicrafts. Arts and crafts | |
| TX - Home economics | |
| U - Military Science | |
| UA - Armies: Organization, distribution, military situation | |
| UB - Military administration | |
| UC - Maintenance and transportation | |
| UD - Infantry | |
| UE - Cavalry. Armor | |
| UF - Artillery | |
| UG - Military engineering. Air forces | |
| UH - Other services | |
| V - Naval Science | |
| VA - Navies: Organization, distribution, naval situation | |
| VB - Naval administration | |
| VC - Naval maintenance | |
| VD - Naval seamen | |
| VE - Marines | |
| VF - Naval ordnance | |
| VG - Minor services of navies | |
| VK - Navigation. Merchant marine | |
| VM - Naval architecture. Shipbuilding. Marine engineering | |
| Z - Books (General). Writing. Paleography. Book industries and trade. Libraries. Bibliography | |
| ZA - Information resources (General) | |
