Beginning with Linux kernel version 4.1 released in early 2015, Linux on Z is only available as a 64-bit operating system compatible with z/Architecture mainframes. Previously Linux on Z was also available as a 31-bit operating system compatible with older model mainframes introduced prior to 2000's z900 model. However, the newer 64-bit Linux kernel and 64-bit Linux on Z distributions are still backward compatible with applications compiled for 31-bit Linux on Z. Historically the Linux kernel architecture designations were "s390" and "s390x" to distinguish between the 31-bit and 64-bit Linux on Z kernels respectively, but "s390" now also refers generally to the one Linux on Z kernel architecture.