A caliper for measuring the size of an object, especially the thickness of an object with non-planar surfaces. After measured, the caliper can be removed from the object without changing the measured value. The caliper includes a scale ruler, a vernier slidably disposed on the scale ruler, a first jaw fixedly connected to the scale ruler and a second jaw pivotally connected to the vernier. In normal state, the second jaw is kept in a position in alignment with and in parallel to the first jaw, whereby the caliper can be used to measure the object in a common manner. After measured, the second jaw is rotated to another position, permitting the caliper to be removed from the measured object.