Sometimes it's useful to see two different sections of the same file at once.
To split a file in Edit mode:
You'll see two different views of the same file, which can be scrolled independently. Since both views represent the same file, changes made in one split will also appear in the other.
Note: Each split can be set to a different mode by clicking one of the six mode icons in the toolbar. This allows you to, for example, edit a file while also viewing its preview.
Note: You can split the tab vertically instead by holding down the option key while issuing the split command.
To change the default orientation for new splits:
To view two different files side-by-side:
Alternatively, you san simply drag a file from the file browser into the desired split.
To close a split:
If the split has unsaved changes, you'll be asked if you wish to save before closing the split.
To resize a split: