How do we change the orientation of the split view?

For whatever reason, DW opens in a top/bottom split view when left/right would clearly be more ergonomic for today's more horizontally rectangular monitors (top/bottom made sense when screens were more 'square' but I don't get the loyalty to the principle today).

But I digress. I've Googled the #### out of this subject and came up short.

How do I place the code view on the left and the design view on the right? I bet it's something really simple, too.