Guide to Tuning the Cello
The 4 strings of the cello are tuned in tonally descending order: a (two keys left of middle c on the piano), d, g, and c. The c string is two octaves below middle c. The a string is the smallest in diameter, and the c string is the largest in diameter. It is vital that the cello be tuned well at the beginning of each practice session. Some cellos stay in tune for a long time, and some need to be tuned more often. There is no use in practicing unless your cello is first tuned up.
How to tune your cello? First, tune the a string to the a below middle c on the piano. Use a piano, or a tuning fork to get the correct pitch. If you are a beginner, just tune your other strings to the appropriate notes on the piano, d,g and c. After you have played for awhile, you will be able to tune by ear using perfect fifths, and you won't need the piano anymore.