# Covariant four-vectors

1. Mar 17, 2014

### spookyfish

Never mind. Please delete this post

Last edited: Mar 17, 2014
2. Mar 17, 2014

### bcrowell

Staff Emeritus
A gradient like $\partial_\mu$ is naturally a lower-index quantity. In the first example you have to raise an index to get $\partial^\mu$.