0: Historical Introduction.- A. The Classical Case.- B. The Non-Classical Case.- 1: Preliminary Results.- A. Preliminary Definitions and Remarks.- B. The General Multiplicity Symbol.- C. The Hilbert Function and the Degree.- D. Miscellaneous Results.- 2: The Main Theorem.- A. The Statement of the Main Theorem.- B. The Join-Procedure.- C. Step I of the Proof.- D. Step II of the Proof.- E. Step III of the Proof.- F. Consequences.- 3: Examples, Applications and Problems.- A. Examples.- B. Applications of the Main Theorem.- C. Problems.- References.