Here is a summary of all the notes you have learned so far.
The next example uses these three new notes along with all the other notes you have learned. Notice that the notes on the sixth string are lower versions of the notes on the first string and require extensive use of ledger lines in the written music.
The Dobro is a metal bodied "Resonator Guitar". It has a resonator cone which amplifies the sound even though there are no electronics involved. There are also wooden bodied guitars with a metal resonator cone. All these guitars are great for slide and fingerpicking. They are traditionally associated with Blues, Bluegrass, Country and other Roots music.