I'm not familiar with tabs and a quick search didn't turn up how to use them. Could you point me to an instruction page? Thanks.
Never mind. I found it: Redirecting
And the relevant stuff:
Multi-file sketches
A sketch can contain multiple files (tabs). To manage them, click on the right-facing arrow just above the scroll bar near the top of the environment. Tabs have one of four extensions: no extension, .c, .cpp, or .h (if you provide any other extension, the period will be converted to an underscore). When your sketch is compiled, all tabs with no extension are concatenated together to form the "main sketch file". Tabs with .c or .cpp extensions are compiled separately. To use tabs with a .h extension, you need to #include it (using "double quotes" not ).