Why Does the Mandolin Use Four Pairs of Strings?

The modern mandolin commonly has eight steel strings, but they do not provide eight independent pitches. They form four courses, each usually tuned as a unison pair, with the courses arranged in fifths—G, D, A and E—like a violin. A plectrum strikes both strings of a course, while the fretting hand treats the pair as one string.

The pairing does not extend the instrument's range; it changes the character of each pluck. Two strings beginning almost together give a small-bodied instrument more weight, while tiny differences in pitch, amplitude and decay add a faint beating shimmer. In tremolo, rapid alternate strokes join many brief attacks into a line that can seem sustained and breathing. The result is not merely louder, but wider and more alive.

There is a cost. If the two strings are not accurately matched, useful animation becomes distracting beating; if the pick catches only one, the attack changes weight. Listen, then, for how unison acquires breadth. The idea resembles a piano's multiple strings for one note, but a piano hammer strikes them in one action, whereas the mandolin player controls each renewed contact. Paired strings are both an acoustic design and a tactile discipline.

https://www.britannica.com/art/mandolin
https://mimo-international.com/MIMO/doc/IFD/MINIM_UK_UEDIN_1292/neapolitan-mandolin
https://www.rosenberg-library-museum.org/treasures/neapolitan-mandolin-from-galveston-mandolin-orchestra