With an electronic tunner, you can also check you intonation. After you have tunned an open string to the correct pitch fret the same string at the 12th fret, and check with the tunner again. It should read the same. If the same string at the 12th fret is sharp or flat, then your string length may need adjustment, or other adjustments may be necessary for proper intonation on your guitar. These adjustments should be done by a competent guitar tech.