The SymbolPair exists to solve the record flattening problem. SqlGenerator.Visit(DbPropertyExpression) Consider a property expression D(v, "j3.j2.j1.a.x") where v is a VarRef, j1, j2, j3 are joins, a is an extent and x is a columns. This has to be translated eventually into {j'}.{x'} The source field represents the outermost SqlStatement representing a join expression (say j2) - this is always a Join symbol. The column field keeps moving from one join symbol to the next, until it stops at a non-join symbol. This is returned by SqlGenerator.Visit(DbPropertyExpression), but never makes it into a SqlBuilder.