To Paul S:
Thank you for your efforts and patience, I do really appreciate it!
The explanation of the #include statement only talks about libraries (<library.h>) and not about the possibility to include a piece of program from a file in another tab ("file.h"). That's why I assumed it should be a library.
I know what a dialog window is, but a window for typing in program statements I would rather call an editing window than a "dialog". I know my description was childischly clear, but I am new to Arduino and didn't want to leave any doubts. When a program is called a "sketch" it could well be that other parts of the IDE have funny names as well.
I found the "arrow-down" button and the menu-item "New Tab"! It never occurred to me that this is a menu-button. Again I downloaded the text of pitches.h from the website and pasted it into the new tab. And now it works!!!!
Thank you all for helping me out. I learned a couple of useful things from this experience.!