ECO CODES
Use these websites to learn the ECO codes of the openings you want to study.
ECO Codes at FICS Games
https://www.ficsgames.org/eco.html
ECO Codes at Chess365.com (with a link to their Chess Openings Trainer for Members)
https://www.365chess.com/eco.php
ECO Codes at Chessgames.com
https://www.chessgames.com/chessecohelp.html