The intonation of a fretted instrument is what makes it play in tune with itself, it depends on the placement of the bridge. To test the intonation play the string open and then play the same string fretted on the 12th fret. If the note sounds sour somehow it means that the intonation is out. An electronic tuner might come in handy for this operation. Move the bridge toward or away from the nut to adjust the intonation. Test each string until you find the spot where each note rings true. It is not uncommon for the bridge on a banjo to sit a little crooked on the head in order to play true, with the treble side being nearer the nut and the bass side being further away.