Cross-walk an industry classification code across the four major systems: NAICS <-> SIC <-> ISIC Rev.4 <-> NACE Rev.2. Pass the system + code and get the equivalent code(s) in every other system, with official titles where available, plus the vintage of each table used. The deterministic join that lets a KYC, procurement, ERP, or cross-border-reporting agent translate "what industry is this" between the US (NAICS/SIC), international (ISIC), and EU (NACE) standards. Backed by bundled public-domain Census concordances (NAICS<->SIC: 1997 NAICS <-> 1987 SIC; NAICS<->ISIC: 2012 NAICS <-> ISIC Rev.4) and the UNSD ISIC Rev.4 <-> NACE Rev.2 correspondence. Multi-hop links (e.g. SIC->NACE via NAICS->ISIC) are derived and explicitly flagged in notes. SIC is a retired US system; there is no newer NAICS<->SIC table.