Merry christmas Raghunathan and all other members:

If I did not know you to be a serious person I would say surely you jest.

Thread mode is the "original " way you get a list of subject lines to select and open. each message needsto be opened to read it.
Flat is where opening an item in the list opens all messages in that thread at once (within reason of course) so you can just read and scroll between mesages to pick up points that you may have missed .
Regards Terry