In standard tuning the strings of the ukulele are tuned to the notes G, , E and A. “G” is the top string; that is, when you hold the ukulele to play, it’s the one closest to your nose and the A string is the one closest to your toes.
The ukulele tuning is known as “re-entrant”, meaning that instead of lower to higher notes like a guitar, the G string is tuned higher than it’s neighbor. In this way, a uke is similar to a banjo!
It’s useful to have a tuner, either attached to the instrument or downloaded on to your phone. Tuners are not expensive; prices range from $5 to $15 for a “clamp on” type tuner (like the Snark-brand tuner shown here). You can download a tuner app- there are dozens to choose from- for free! In the case of either the app or the physical tuner, they work roughly the same. The tuner will recognize when the target tone is close to the value you are looking for. It will give you a visible indication that your tuning is proper. When you are flat (meaning the string needs to be tightened to make the sound higher) the indication will be to the left of center. If the string is sharp, or tuned too high, the indicator will be to the right of center. When the tuning is correct, generally the indicator will be centered and will change color to tell you your tuning is correct. There are plenty of on-line resources for tuning your ukulele! Here’s a PDF of this material and below is a video that I made that might be helpful:
