What word set completes the analogy to chat and talk?

This is a bit tricky because "chat" and "talk" are very close in meaning. They both refer to verbal communication.

Here are some options depending on the nuance you're looking for:

* Informal vs. Formal: If you're thinking about the formality of the communication, you could use chat : talk :: gossip : discussion.

* Length and Depth: If you're thinking about the length and depth of the conversation, you could use chat : talk :: casual : serious.

* Number of Participants: If you're thinking about the number of people involved, you could use chat : talk :: one-on-one : group.

Let me know if you have a particular aspect of "chat" and "talk" in mind, and I can give you a more specific answer!