A7
Notes: A – C# – E – G — 1 – 3 – 5 – ♭7 (dominant seventh chord)
The added minor seventh (G, instead of A major's G#) creates the characteristic dominant tension: in the key of D, A7 is the V7 that pulls the ear toward resolving on D. It's also the I chord of a blues in A, where that same tension is left deliberately unresolved for the whole tune.
Every sheet in the catalog that uses the A7 chord, across all styles and levels. The same chord comes back in very different contexts — a blues grid, a pop turnaround, an arpeggio sequence: a good way to review it without always playing the same thing.