Exploring isotropic negative thermal expansion (NTE) compounds is interesting, but remains challenging. Here, a new concept of "average atomic volume" is proposed to find new NTE open-framework materials. According to this guidance, two NTE compounds, AgB(CN)(4) and CuB(CN)(4), have been discovered, of which AgB(CN)(4) exhibits a large NTE over a wide temperature range (alpha(v) = -40 X 10(-6) K-1, 100-600 K). The analysis by extended X-ray absorption fine structure spectroscopy and first-principles calculation indicate that (i) the NTE driving force comes from the transverse vibrations of bridge chain atoms of C and N, corresponding to the low-frequency phonon modes; and (ii) the same transverse vibration direction of C and N atoms is a key factor for the occurrence of strong NTE in AgB(CN)(4). The present concept of "average atomic volume" can be a simple parameter to explore new NTE compounds especially in those open-framework materials.