A block is a unit measuring the number of bytes that are read, written, or converted at one time. Command-line options can specify a different block size for input/reading () compared to output/writing (), though the block size () option will override both and . The default value for both input and output block sizes is 512 bytes (the traditional block size of disks, and POSIX-mandated size of "a block"). The option for copying is measured in blocks, as are both the count for reading and count for writing. Conversion operations are also affected by the "conversion block size" ().