Why is a 3-way merge advantageous over a 2-way merge?

Question

Wikipedia says a 3-way merge is less error-prone than a 2-way merge, and often times doesn\'t need user intervention. Why is this the case?

An example where a 3-way

Answer 1

This slide from a perforce presentation is interesting:

The essential logic of a three-way merge tool is simple:

• Compare base, source, and target files
• Identify the "chunks" in the source and target files file:
  • Chunks that don't match the base
  • Chunks that do match the base
• Then, put together a merged result consisting of:
  • The chunks that match one another in all 3 files
  • The chunks that don't match the base in either the source or in the target but not in both
  • The chunks that don't match the base but that do match each other (i.e., they've been changed the same way in both the source and the target)
  • Placeholders for the chunks that conflict, to be resolved by the user.

Note that the "chunks" in this illustration are purely symbolic. Each could represent lines in a file, or nodes in a hierarchy, or even files in a directory. It all depends on what a particular merge tool is capable of.

You may be asking what advantage a 3-way merge offers over a 2-way merge. Actually, there is no such thing as a two-way merge, only tools that diff two files and allow you to "merge" by picking chunks from one file or the other.
Only a 3-way merge gives you the ability to know whether or not a chunk is a change from the origin and whether or not changes conflict.