A mandolin chord
A major on mandolin is 2-2-4-(5), low string to high (G D A E), and it sounds A E C# (A). 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.
First finger lies flat across the G and D pairs at the second fret, third finger takes the A pair at the fourth, and the fourth finger reaches the E pair at the fifth when your hand allows it. That top course doubles the A you already have on the bottom, so the chord is whole on three.
Nothing here touches an open string, so nothing holds the shape where it is. Slide the whole grip up two frets and the same four fingers give you the B on this chart. Drop the A pair a fret instead and the C# becomes C, which is the third that turns it minor.
Where it turns up
The I chord in A, the IV chord in E, and the V chord in D. 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.