Vendors of some proprietary Unix systems, such as SunOS / Solaris, System V Release 4, HP-UX, and Tru64 UNIX, and open Unix derived systems like illumos, have adopted UFS. Most of them adapted UFS to their own uses, adding proprietary extensions that may not be recognized by other vendors' versions of Unix. Many have continued to use the original block size and data field widths as the original UFS, so some degree of read compatibility remains across platforms. Compatibility between implementations as a whole is spotty at best.