Key strata is a rock seam which plays an important controlling roles in the movement of mining overlayer strata while the key strata is sometimes made up of two or more compound rock strata 摘要在采场覆岩运动中起主要控制作用的岩层为关键层，而有时关键层是由2层或2层以上岩层形成的复合岩层所组成，我们称其为复合关键层。
