A calibration process always involves a comparison between:

The calibration process fundamentally relies on the principle of comparison to ensure accuracy and reliability. In calibration, the goal is to determine the accuracy of a measuring instrument by comparing its measurements against a standard that is known to be accurate. This standard is referred to as a "known" item, while the device being calibrated is considered the "unknown."

By using a known item, such as a reference standard that has a predetermined and validated measurement, technicians can assess the performance of the instrument. This process allows them to identify any discrepancies between the readings of the measuring device and the known value, which is critical for ensuring that measurements taken by the device are precise and reliable.

In contrast, comparing two unknown items does not provide a baseline for accuracy, as there is no definitive standard to reference. Evaluating different models of the same item might help in understanding performance characteristics, but it does not strictly conform to the calibration process as it lacks a clear comparison to a known standard. Moreover, theoretical values may not always be practical or applicable in a real-world setting and thus cannot ensure calibration accuracy without a known reference value.

Ultimately, the act of comparing a known item with an unknown one is what defines the calibration process and ensures the integrity of measurements in various fields.