Airlines cannot meet to discuss fares for the most part (some airline alliances have partial exemptions).

Airlines do copy fare increases. Sometimes they don't which can lead the original fare increase to be discontinued.

Airlines have been caught sending messages to each other. About 15 years ago or so, airlines used the fare basis to send a message. For example, if United was unhappy with a fare increase by Delta, they would have a promotional fare on a route to ATL and call it something like YDL3, meaning Y for coach, DL for (expletive) Delta, stop this. Or maybe it was even less subtle like YDLIAHLAX for Delta, stop that IAH LAX fare?

Anyone know the details of how the messages were coded?