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The Standard Industrial Classification system. A legacy four-digit numerical code structure developed in the late 1930s to categorize business activities, still used by SEC EDGAR and OSHA.

• NAICS vs. SIC vs. ISIC: Understanding Industrial Classification Systems — A comparative analysis explaining the relationship, scope, and differences between NAICS, legacy SIC, and the international UN ISIC standard.
• SIC Codes Explained: History, Structure, and Concordances — The complete history and structural layout of the 1987 Standard Industrial Classification (SIC) system, including its legacy role.
• Why Legacy SIC Codes Persist in Modern B2B Databases and Systems — Explains why four-digit SIC codes remain widespread in marketing data, risk underwriting, and historical datasets despite being legacy.
• SIC ↔ NAICS Concordance Crosswalk — Search and map old SIC codes to modern formats.

1. U.S. Occupational Safety and Health Administration (OSHA): Standard Industrial Classification (SIC) Manual
2. U.S. Securities and Exchange Commission (SEC): SEC EDGAR Filer Standard Industrial Classification Codes