Same tack, overlapped, and no rule of Section C or D applies instead.
If you are side by side on the same tack, the boat to windward gives way. Being overlapped means neither boat is clear astern of the other: draw a line square across the stern of the boat ahead, and if any part of the other boat's hull or equipment in normal position is forward of it, they overlap.
The windward boat must stay far enough away that the leeward boat can sail her course and also turn either way without immediately touching. That second limb of keep clear matters: a windward boat that sits so close that leeward cannot luff at all is not keeping clear, even if leeward is sailing straight.
The leeward boat's freedom is limited by rule 15 when she has only just become right-of-way boat, by rule 16 when she changes course, and by rule 17 if she became overlapped from clear astern within two hull lengths.
Common situations
Luffing to windward
A leeward boat with luffing rights may luff, even head to wind, provided she gives the windward boat room to keep clear as she does it (rule 16.1). A slow, steady luff is fine; a sudden swing that windward cannot answer is not.
Before the start
There is no proper course before the starting signal, so a leeward boat that established her overlap from ahead or by tacking can luff to head to wind while she gives room. Windward boats on the line get caught by this every season.
Common misreadings
"The windward boat only has to avoid hitting"
The windward boat must leave leeward room to change course in both directions without immediate contact. A windward boat whose hull or boom is over leeward's deck is almost never keeping clear.