What is it? Despite the name, "Excel 91" is not a single built-in function. Instead, it refers to a powerful two-formula combination : =OFFSET(...) combined with =MATCH(...) . It earned the nickname "Formula 91" in某些 training communities (possibly a mnemonic for "9 out of 10 complex lookups") because it solves lookup problems that VLOOKUP and INDEX-MATCH cannot handle easily. Core Syntax =OFFSET(reference, ROWS to move, COLUMNS to move, [height], [width]) =MATCH(lookup_value, lookup_array, [match_type])