ftp and ftp-data

Hi all, just a dumb question: is the port for ftp-data always ftp-1 ? I mean: usually ftp == 21 and ftp-data == 20, so if I start a FTP server on, say, 50021, would ftp-data be 50020 by default? if not, where do I configure that ftpd's behaviour? TIA and regards, Martin