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

A three way merge where two changesets to one base file are merged as they are applied, as opposed to applying one, then merging the result with the other.

For example, having two changes where a line is added in the same place could be interpreded as two additions, not a change of one line.

For example

file a has been modified by two people, one adding moose, one adding mouse.

#File a
    dog
    cat

#diff b, a
    dog
+++ mouse
    cat

#diff c, a
    dog
+++ moose
    cat

Now, if we merge the changesets as we apply them, we will get (3-way merge)

#diff b and c, a
    dog
+++ mouse
+++ moose
    cat

But if we apply b, then look at the change from b to c it will look like we are just changing a 'u' to an 'o' (2-way merge)

    #diff b, c
    dog
--- mouse
+++ moose
    cat