端基差向异构又称为首旋异构物，一般存在于糖类中，是差向异构的一种，两个非对映异构体分子（异头物）的差异在于糖类环形结构半缩醛/半缩酮碳原子（异头碳）的构型不同。1号碳的羟基若与5号碳的羟甲基处于哈沃斯透视式平面的两侧，则定义为α-异构体，反之称为β-异构体。吡喃葡萄糖的两种端基差向异构体可分别称为“α-D-吡喃葡萄糖”和“β-D-吡喃葡萄糖”。

α-异构体与β-异构体的互相转化称为变旋现象。α-异构体受端基异构效应影响而得到稳定。

An anomer is a type of stereoisomer and epimer found in carbohydrate chemistry. While an epimer is a stereoisomer that differs in configuration at any single stereogenic center, an anomer is a cyclic saccharide and an epimer that differs in configuration, specifically at the hemiacetal/acetal carbon, also called the anomeric carbon. The anomeric carbon is the carbon derived from the carbonyl carbon (the ketone or aldehyde functional group) of the open-chain form of the carbohydrate molecule. Anomerization is the process of conversion of one anomer to the other.