If you have access to a piano, you can use the keyboard to play the pitch references.
Middle C
First locate the note middle C on the keyboard; this note C is very near the middle of the keyboard, and on most pianos, is near the makers label on the front face board. Once you find middle C on the piano, all of the other C's can be easily seen, as the keyboard has a symmetrical design with the layout of the keys following pattern.
String Number 6
To play the pitch E on the keyboard that will match string number 6 open on your guitar, start from the middle C and find the pitch C that is exactly one octave lower than middle C.
From this C, find the C that is exactly one octave lower again.
From this low note C, move up two more white keys and play the pitch E.
This is the pitch E on the piano that matches string number 6 open on your guitar.
Find the other pitches for strings 5,4,3,2,and 1 in a similar way, using middle C as a reference point.
Timbre
Because the piano and the guitar are made differently, the notes each produces will have a different timbre ( pronounced tem-ber) or tonal color, so when you compare the two tones, try your best to focus only on the pitch, not allowing the timbre to confuse your ear.
A tone that sounds thin or bright in timbre could be mistaken for a higher pitch, and a tone that sounds thick or warm in timbre could be mistaken for a lower pitch. Using your voice and humming the notes in turn may help you better focus on the pitch. With practice in listening, you will improve your ability to separate the qualities of a tone and focus on any one quality at a time.
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