(1) True [[bed]] thickness is the thickness of the stratigraphic unit measured along a line normal to the direction of extension of the unit. True thickness can be derived from information determined by the [[dipmeter]].

(2) The bed thickness determined from some [[well log|well logs]] is an apparent bed thickness corresponding to the distance the borehole remained in the bed. The borehole may not have penetrated the bed normal to its upper or lower boundary surface because of hole [[deviation]] and formation [[dip]]. See also [[true vertical depth]].