A synthesis is a breeding of two monsters resulting into one new monster, both monsters used in the synthesis are lost. Monsters are divided in charges (+), (-) and (+/-).Only monsters of the opposing charge can be synthetised, (+/-) monsters act as neutrals and can be synthesized with both (+) and (-). The stats of the new monster are determined this way: (Monster1 Stats + Monster2 Stats)/4. Most of the Rank S and X monsters can only be synthesized. There is something called Quadrillineal Synthesis, where the grandparents of the two monsters you are synthesizing are important in the synthesis and the parent monsters do not matter, this is the only way to get monsters like Mumbo-Jumboe and Don Mole.