You forgot to make it public, dummy!

Microsoft has a convoluted way of saying this;

Error 1 Inconsistent accessibility: property type 'System.Collections.Generic.List' is less accessible than property 'Frets.MusicalScale.notes' J:\Users\Larry\Documents\Visual Studio 2008\Projects\Frets\Frets\Classes\MusicalScale.cs 44 41 Frets