An armónico is a guitar-like instrument. It is a hybrid between the Spanish guitar and the Cuban tres. Formally known as trilina, the armónico has seven strings, two of which form a doubled string. That's six courses, five with one string, and one with two strings.
Counting from the lowest-pitched, the first three strings, E, A and D, are tuned an octave higher than the equivalent strings on a guitar with standard tuning. The fourth string, G, is doubled as well as also being an octave higher than on a guitar. The remaining two, B and E, are at the same pitch as a standard guitar.