什么是 Classes of folds？

By its interlimb angle, a fold is a gentle fold ($\iota$ from $180{}^{\circ}$ to $170{}^{\circ}$), an open fold ($170{}^{\circ}$ to $90{}^{\circ}$), a tight fold ($90{}^{\circ}$ to $10{}^{\circ}$) or, when its limbs are nearly parallel ($\iota$ from $10{}^{\circ}$ to $0$), an isoclinal fold. By the dip of its axial plane, a fold is an upright fold when the axial plane dips more than $80{}^{\circ}$, a recumbent fold when it dips less than $10{}^{\circ}$, and an inclined fold in between. A fold one of whose limbs has turned past the vertical, so that both limbs dip the same way and the beds of that limb are upside down, is an overturned fold. A fold whose hinge line is not horizontal is a plunging fold.