B mandolin chord
B major on mandolin is 4-4-6-(7), low string to high (G D A E), and it sounds B F# D# (B). It is one of the core chords on the chart.
The shape
Nut at the top, strings running down the page low to high, G D A E. The numbers under the name are the frets in that order. A filled dot is a finger and a hollow dot is the string you add when your hand allows it. An o above the nut is an open string, and a dashed o is an open string you can leave out.
The same grip as the A: first finger flat across the G and D pairs, this time at the fourth fret, third finger on the A pair at the sixth, and the fourth finger on the E pair at the seventh when you can reach it. There is no fingering to relearn, only a hand two frets further along.
There is no nut in the diagram, so the grid numbers the top fret of the window instead. That is the tell for a shape that moves. Carry the same grip to the seventh fret and it is a D, to the ninth and it is an E.
Where it turns up
The I chord in B, the IV chord in F#, and the V chord in E. Those three keys are where you will be asked for it.
Tunes that use it
Written out here with the chord letters over the bars where the change happens, so you can put the shape under something rather than drilling it on its own.
A diagram cannot hear you
Mandolin Trainer listens through your microphone, hears which notes of the chord are sounding, and names the string that should be carrying a note it cannot hear, which is the thing a diagram can never do. The Chord Theory section teaches these shapes and then checks them. Free while it is in beta.
Open the appRuns in your browser. The audio never leaves your device. iOS app coming soon.
Other core chords
The seven a beginner needs first. The first three ring against open strings, and the rest are closed shapes that move.