Over the last two decades of biometric research, it has been shown in numerous occasions the key impact that the quality of biometric samples has on the performance of biometric recognition systems. Few other biometric characteristics, if any, have been analysed so in depth from a quality perspective than fingerprints. This has been largely due to the development by the US NIST of two successive systemindependent metrics that have become a standard to estimate fingerprint quality: NFIQ1 and NFIQ2. However, in spite of their unquestionable influence in the development of fingerprint technology, there is still a lack of understanding of how these two metrics relate to each other. The present article is an attempt to bridge this gap, presenting new insight into the meaningfulness of both metrics, and describing a mapping function between NFIQ2 values and NFIQ1 classes.