Should I Get Medium Strings? Lights? Heavies? Will it change my tone?

To start off with a generalization, most players use mediums, in my experience selling strings. Does that matter to you? Probably not.

Some strings come in only one weight, but quite a few have multiple tension options. What's the diff? Simply, light strings have slightly less tension than mediums, and heavy strings have slightly more tension. Specifically, the strings have more or less mass, to require a few pounds more or less tension to create the specific pitch desired, which makes the strings feel "tighter" or "looser."

Those that offer additional string tensions usually designate the "standard" gauge as "Medium" (or "Mittel" if they're certain European brands), and then a "Light" ("Weich" or "Dolce") and/or "Heavy" ("Stark" or "Forte") gauge. Though, some manufacturers designate them differently; Corelli's 370 Strings come in "Medium," "Forte (heavy)," and "TX/Extra Forte (extra heavy)," while Eurosonics come in "Medium," "Light," and "Ultra Light." And, just to be different, some string makers designate their medium strings as "Orchestral" gauge. What does that even mean?

Yeah, that's what we need - more confusion.

How Different Are the Various Weights?
Generally, each of the various tension "versions" of a particular string model are manufactured in pretty much the same way, but with more or fewer windings to change the mass, and therefore the tension. However, it is also true that sometimes the manufacturer will alter the design or the ratio of some ingredients to achieve the perfect balance; for instance, Corelli 370 strings may have varying levels of tungsten steel in certain strings to get the set to have the appropriate tonal and volume balance, depending on gauge.

Why Should I Change?
Reasons to change gauges? That's very subjective. The biggest difference is in playability. A heavier string will allow for a heavier plucking hand and will be more physically demanding to play; a lighter string will be easier on the left (aka fingering, sorry left-handers!) hand. Some players prefer a lighter feel, while others like the strings to be harder, because it suits their playing style to pluck with more vigor. Some people can play amplified and let the amp do some of the work for them, while others people play acoustically on Bourbon Street as loudly as they can. Some people find that lighter strings bow more easily, while others play rockabilly slap-style on the weekend and need a lighter string for more "bounce." Some feel that their tone comes from a light, deft touch – others beat the bass like it owes them money.

Using "Solo" Strings as Lights
Some sets do not have "Lights" but may have a "Solo" set, which is designed for tuning up one whole step, which can be tuned to standard pitch and played as if it were a light string (Thomastik Superflexibles and Pirastro Obligatos are two popular examples.) Here's some more information about Solo Gauge Strings.

My Bass is Really Old. Should I Get Lights to "Take It Easy" on the Bass?
Hmmmm... Here's the thing. If your bass has had a bunch of reconstructive surgery, like a shattered and rebuilt top, and your luthier says to use light strings - I defer to the luthier. But if you have an old Kay or King bass, or something similar, and it's structurally sound with no apparent issues, I see no reason why you can't put whatever strings you want on it. The bass was designed to hold that tension, and unless there is something specific about that particular instrument you're worried about, you should be able to put medium, or even heavy, strings on it, even if it's "old." On a side note that's related, there should be no reason to de-tune or "slack" the strings if you don't plan to play the bass for a week or two (or even longer) - I've heard of people doing that, and quite frankly, I personally believe that it's probably worse for the bass (and definitely the strings) to alter the tension back and forth, not better.

Tonal Differences
All else being equal, strings of different weights do usually sound pretty similar. However, there can be subtle tonal changes associated with different weights of the same string design.
It's my understanding that most string makers that offer various tension weights (light/weich/dolce, medium/mittel, heavy/stark/forte) generally do so with only the intent of having the tension be higher or lower to suit a particular playability preference. It's not (again, as I interpret it, there may be exceptions) to create different tonal "colors."

However, since the means for offering alternate tensions involves (usually) fewer or additional layers of windings to create more mass, and in some cases, a slightly different formulation (often to maintain string balance, as in the case of Corelli 370 strings), it's certainly plausible that a player might pick up some other (unintentional) differences besides tension, all else being equal.

Additionally, the pressure of the strings on the table of your instrument will increase or decrease depending on the strings, and that can have a (subjectively) positive or negative affect on both the tone and the volume of the instrument. For some folks, lighter strings "wake up" the instrument, because it allows the top to vibrate more effectively. Other basses can benefit from a string that "bears down" more heavily on the top and "drives" it to better, fuller sound.

That said, heavier strings are usually less “elastic” and can have a subtly more full-bodied sound. A stiffer string will allow for playing with a heavier hand, and this can marginally increase volume and "power." Lighter strings can sometimes create more “growl” as they tend to lower the action (with the overall lessened tension) and they vibrate more widely, which causes them to interact with the fingerboard a bit more. But again, you’re talking about a small difference; the sound of the strings is generally about 95% alike (if you’ll permit me to guesstimate for the sake of putting it in simple terms.) Setup and tone color of every individual bass is different, and the minor changes from gauge to gauge may be too subtle to notice on your particular instrument, or with your particular playing style. 
It's definitely possible.


The TLDR; Section
Put simply, everyone’s journey is personal, and there’s no “right” string for everyone. Play the string that you're comfortable with, and don't worry what's on the package. Playing a light string isn't being a "sissy." Look, bass playing isn't like Olympic diving; you don't get extra points for degree of difficulty.

An Experiment

Want to get a general idea of what a light or heavy version of your current string set might feel and sound like? The way to feel the approximate effect of the tension difference would be to do this: De-tune your medium strings by approximately 3/4 of a step (tune the G string down to about halfway between F and F#, and so on down all four strings). Now play; this will approximate what the lights would probably feel and sound like (at pitch). Conversely, tune the strings up about 1/2 step to get the approximate feel of the heavy strings.

Now, remember that every string set's construction is a bit different, and results may vary; this is a very general way to sort of "preview" what the other gauges could feel and sound like on your bass, but it's not exact.

More Help
If you're trying to figure out whether you should choose light, medium, or heavy (if available), we should address it on a micro level - and try to figure out which strings might be best FOR YOU. Can you describe the silked ends for me? Maybe compare them to the (limited) ones we have shown at this other FAQ?

If we can figure out what’s on the bass, you can tell me what you DO and DON’T like about them, and we can try to figure out the right string (and gauge) to suit your needs.