Gauge, material, type of winding, coated or uncoated...shopping online for strings or at your local music store can be a daunting task for the uninitiated. Here are some helpful definitions and suggestions that will help you narrow down your options.
The gauge of a string is its thickness (diameter). Presented in thousandths of an inch, when it comes to bass strings you'll see measurements like .045 (G), .065 (D), .085 (A) and .105 (E). To simplify your choices, many manufacturers use terms for sets like "Medium," "Medium Light," "Light," etc. These terms aren't necessarily universal—for example, Ernie Ball's "Medium" might not be the same gauges as Dean Markley's. If you settle on a single brand of strings, like many players do, it will make it easier for future purchases. What you need to pay attention to is the feel you're after, the sustain and the fatness of the tone you get from different gauges. As you might expect, heavier-gauge strings can be harder on your fingers if you play fingerstyle, and they can also make a difference to the playability of your instrument. Keep in mind that if you move to heavier strings, your bass will likely need some adjustment. So don't just go from light gauge to heavy gauge without making these adjustments, or you'll likely encounter buzz, intonation and neck trouble. Bottom line: gauge is all about finding the right balance between playing comfort and a tone you're happy with. Adjust as necessary.
The scale length of your bass is the distance from the bridge to the nut, or the portion that vibrates and produces sound. Most basses have a long scale of 34", however many basses utilize a shorter, 30" scale as well. These are typically student-sized models, but many players have also employed short scale basses for their unique feel and sound. It's important to know the scale length of your bass when purchasing strings, so be sure to look it up or do a quick measurement from the bridge to the nut to be sure.
Bass strings consist of two parts: the core and the winding. These can be, and often are, different materials. Typically the core is made of steel, and can either be round in shape or hexagonal (hex core). A hex core provides for a more consistent winding tension, enhancing sustain and providing better intonation. Round cores are not as common these days, and are associated with more of a "vintage" sound.
Now we get to the all-important winding. If you look at your strings, you'll likely notice ridges all along the length. This indicates a string that is "roundwound," and these are by far more common than their counterparts, "flatwound" strings. The difference is evident in their names--roundwound strings use a rounded wire to wrap the core, while flatwound strings use a flat ribbon of metal around the core, resulting in a string that is smooth to the touch. Tonally, flatwound strings produce little to no extraneous noise as your fingers slide up and down the neck, and aren't as bright as roundwound strings as a result. Because there's less friction, they are also easier on your fingers and your instrument. Although not as popular as brighter-sounding roundwound strings anymore, some players still swear by them. I recommend at least trying out a set of flatwound strings to see if you fall in that camp, but if you're a fan of a bolder, more in-your-face sound (or have to compete with loud guitars), roundwound strings are likely what you'll want. Though flatwound and roundwound are the most common, there are other types of wraps too. Groundwound (AKA half round) strings are a sort of hybrid between flat and roundwound strings, offering a similar feel to flatwound while retaining the tonal brightness and overtones of a roundwound string.
The material for the winding can vary, with each having an influence on the magnetic interaction with your pickups and the tonal end result. Some of the more popular windings you'll come across are nickel-plated steel, nickel and stainless steel, but there are others as well, like D'Addario's EXP REDS that use a copper-plated steel winding that imparts more of an acoustic tone due to the way it interacts with your pickups. By knowing the tonal characteristics of wrap materials, you can better find a string that will suit your sound. Here's a quick overview:
Nickel-plated steel: This common winding produces the bright, lively sound that many bassists prefer.
Pure nickel: Pure nickel windings produce a softer, mellower tone. Often associated with more of a "vintage" sound.
Stainless steel: Stainless steel strings are very bright and responsive to your attack. Due to the hardness of the metal, stainless steel-wound strings can be harder on your frets and your fingers.
In an effort to increase the longevity of bass strings and improve the feel, many manufacturers offer coated strings. These coatings are typically a proprietary compound specific to each company, so they can vary in their feel and effectiveness. Many coated strings do successfully lengthen the life of the strings, however the tradeoff is usually a silky or slippery feel. It's something that many players adjust to in order to save money over the long haul. It's worth trying out a set or two of coated strings to see if they're up your alley.
Hopefully this primer gives you a better idea of the types of bass strings you'll encounter and what each has to offer. As with most aspects of your musical development, experimentation is key, so try a few different types and gauges until you find the strings that are right for you.
Ara Ajizian, Harmony Central's Editorial Director, has been playing bass and guitar as well as singing since he was 18, and soon that love of music combined with a passion for writing; launching what's now a decade-long career immersed in the gear world. He's thrilled to be back on the Harmony Central team after two years as Managing Editor for Musician's Friend covering gear, bands and events and gigging in the Los Angeles area.