Y-Modem is a character-oriented communication protocol for secure file transfer. In contrast to the predecessor protocol X-Modem, Y-Modem works with automatic block length adaptation, whereby the block length
changes depending on the transmission quality. As with the more powerful variant of X-Modem (X-Modem 1K), the block length is 1024 bytes
. The Y-Modem protocol was used in the 1980s as a transfer protocol between mailboxes and personal computers. It was used to transfer file names and attributes, allowing the simultaneous transfer of multiple files. The Y modem was later replaced by the Z modem.