Because there are so many different for thermoforming sheet, the basic forming methods will be reviewed in this section. Most of the illustrations (Figs. 12-2 through 12-15, inclusive) will be based on equipment incorporating paralel-acting frames, heaters above and below the sheet stock (called sandwich heaters and capable of heating both sides of the sheet), male mold and plug assists above the stock (except in the case of drape and slip forming where the male mold is below the stock), and female mold below the sheet.