The construction of a new high‐pressure cell is presented. It is a piston‐cylinder type of apparatus. The entrance and exit of X‐rays or light are possible through two parallel diamond‐plate windows placed on V‐shaped slits covering almost the whole range of reflection angles. This makes the cell particularly suitable for X‐ray diffraction examinations of monocrystals. The potential accuracy of absolute‐bulk‐modulus determination by lattice‐constant measurements using the Bond method [Bond (1960).Acta Cryst.13, 814–816] at pressures of 108–109Pa can be estimated at 1–2%. The cell enables the simultaneous examination of two samples, which makes possible the comparison of their compressibilities with an accuracy of 0.01–0.1%. As examples, the results of compressibility measurements