Gauge is the only decision that changes your hands
String gauge is quoted as the thickness of the thinnest string in thousandths of an inch. A "light" acoustic set is usually .012 to .053; an "extra light" is .010 to .047. Thicker strings need more tension to reach the same pitch, and that tension is what your fingertips are pushing against on every single note.
D'Addario publishes the tension of each string in pounds. Their light EJ16 set totals 160.5 lb across six strings. That figure is not impressive on its own — its value is that it is comparable, and almost no other brand gives you anything to compare with. Elixir, Ernie Ball and Savarez all publish gauges and no tension. The gauge guide lays the numbers out in a table.
For a first acoustic, light (12s) is the default and extra-light (10s) is the answer if your fingertips are still raw after a week. For a first electric, 9s are kinder than 10s and you will bend notes months earlier because of it. Neither choice is permanent; a set of strings is a reversible decision.
Coating is a lifespan decision, not a tone decision
Uncoated strings die from two things: the oil and salt off your hands, and the gunk that packs into the gaps between the windings. A coating — Elixir's NANOWEB, D'Addario's XT — seals the string so both take longer to happen.
The trade is real in both directions. Coated sets stay bright for weeks rather than days and squeak less under a sliding hand. They also cost several times as much, and they never have the raw zing of an uncoated string on day one. If your hands sweat, or the guitar sits out for weeks between sessions, coated sets pay for themselves. If you play daily and change strings monthly anyway, they do not. That argument in full: coated vs uncoated.
Acoustic, electric and nylon are not interchangeable
Acoustic strings are bronze or phosphor bronze over a steel core; the bronze is what a wooden top responds to. Electric strings are nickel or nickel-plated steel, because a magnetic pickup can only hear a magnetic string — bronze strings on an electric produce almost no output.
Nylon strings belong on classical guitars built for them. Putting steel strings on a classical guitar can pull the bridge off, because the neck and top were never built for that tension. Putting nylon on a steel-string acoustic sounds dead and fits the bridge badly. Check what your guitar was designed for before buying anything.
When to change them
The honest answer is: when they stop sounding right or start feeling wrong under your fingers, whichever comes first. Dull tone, dark discoloration on the windings, a rough gritty feel under a sliding finger, or one string that will not hold its tuning — any of those is the signal. How often to change strings works through the actual variables, and how to change guitar strings is the step-by-step for doing it the first time without dropping a bridge pin inside the guitar.
