Commit only part of a file in Git

Question

When I make changes to a file in Git, how can I commit only some of the changes?

For example, how could I commit only 15 lines out of 30 lines that have been changed

Answer 1

Tried out git add -p filename.x, but on a mac, I found gitx (http://gitx.frim.nl/ or https://github.com/pieter/gitx) to be much easier to commit exactly the lines I wanted to.