Spend a little time at your friendly local guitar shop or lurking in online forums and you're likely to run into a bass player with a strong opinion, one way or the other, about flatwound strings. String wind is one of those things - like using a pick vs playing fingerstyle - that bassists tend to get a little tribal about, turning their personal preference into an aesthetic decree. Real bassists do it my way. Everything else is garbage.
They aren't exactly wrong. Round wound and flat wound strings each have a distinctive sound - and feel. Like the picking versus plucking debate, there's no objectively "right" way to do it, but it's not just a question of a personal preference. The way that your strings are wound does matter.
So what are flatwound strings, and why would you want to use them?
Guitar String Construction
Bass guitar strings - and most guitar strings - are a little more complex than you might think if you've never examined them closely. Only the lightest-gauge strings - like the B and high-E strings on a 6-string guitar - are a simple, single wire. Practically all other electric guitar strings use a compound construction: typically with an outer "wrap wire" wound tightly around a central "core wire". These are known as "wound strings".
The shape of the outer wrap wire is difference between rounds and flats (and more esoteric variations like half-round and ground-round strings).
Flatwound vs. Roundwound Construction
The outer (wrap) wire on roundwound strings has a circular cross-section. That is, if you were to slice your guitar string the long way - down the length of the core wire and across each coil in the wrap wire - a roundwound string would look something like this:
The circular cross-section of the wrap wire on roundwound strings creates small bumps or ridges that you can both see and feel. Look closely at round wound string - or run your fingernail along its length - and you'll easily notice the little "valley" between each individual ply of the wrap wire.
A flatwound string is made with an outer (wrap) wire that is, well, flatter. Seen in cross-section, the wrap wire found on a flatwound string is more like a rounded rectangular than a perfect circle.
The beveled shape of the outer wire on flatwound strings fills in the gaps between individual coils of the wrap wire. There's a shallower "valley" between each ply of the wrap wire, which makes the ridges or bumps between coils much less noticeable on flatwounds.
Flats vs. Rounds
If you bought your bass from a shop or online, rather than a used guitar that someone may have already customized, it almost certainly came with roundwound strings. These are by far the most common type - not least because rounds tend to be less expensive than flats - but more importantly because round wound strings are versatile, expressive, and well-suited for almost any style or genre.
For this reason it's convenient to think of rounds as a sort of generic or default string - a baseline we can use to describe the characteristics of flatwound strings.
The Flatwound Feel
Imagine the squared-off cross-section of flatwound wrap wire as a slightly "squished" version of the circle found in the cross-section of a roundwound wrap wire.
This changes the feel of flatwound strings in a few notable ways:
The smaller, shallower grooves between individual plies (coils) of the wrap wire on flatwound strings are nearly imperceptible to touch. This gives flatwounds a very smooth texture, almost as if the string were made of one continuous ribbon rather than a wrapped wire. Sliding your finger along the string feels a bit more "fluid" on a flat-wound string.
The wrap wire on a flatwound string is typically slightly thinner than the wrap wire on a roundwound string. That means that for a given string gauge (thickness) the core wire on a flatwound string will be slightly thicker - and therefore stiffer - than the one found in the corresponding roundwound string. Flat-wound strings may feel a little stiffer than their round-wound equivalents - requiring a slightly more pressure to pick or pluck.
Notice that the gap between coils of the wrap wire is smaller on the inside as well as the outside of flatwound strings. A much higher proportion of the length of core (inner) wire is in contact with the wrap (outer) wire on a flatwound string. This also contributes to flat wound strings feeling a little stiff compared to rounds.
The Flatwound Sound
Roundwound strings are characterized by a fast attack and slow decay. A firm pluck generates a crisp tone that rings out brightly.
Flatwounds are warmer and more muted in comparison. Some of the more commonly noted characteristics of the flatwound sound are:
The relative stiffness of flats generates a tone that is more "pure", emphasizing the fundamental note over harmonic overtones.
Their tone isn't as bright, especially in the treble spectrum. Haters might say flats sound "dead". But flats shine the middle and low end.
The ribbon-like surface - and a less aggressive attack - encourages slower, smoother transitions between notes.
The shallower grooves between coils of the wrap wire are less likely to accumulate dirt and grime (on both the inside and outside of the string). This makes flat strings less susceptible to the kind of degradation that creates "dead" rounds - they last longer and maintain a more consistent tone over their lifespan.
The flowing, mellow sound of flatwound strings has made them a virtual standard for jazz guitarists, and popular with bassists across many genres.
Examples of the Flatwound Sound
So that you can hear the difference for yourself, the following video includes a head-to-head comparison between roundwound and flatwound strings. The same piece of music is played four ways: picked and plucked (fingerstyle), with round-wound and flat-wound strings.
You can use the following links to jump to specific timestamps in the video and hear the various guitar set-ups:
For more information about flatwound guitar strings and related topics, visit the following articles from FATpick and around the web.
How To Choose Guitar Strings When You Are In A Hurry - TL;DR-style summary of everything you need to know to buy guitar strings with confidence
Sweetwater: Flatwound vs. Roundwound Bass Strings - Another introduction to flat-wound strings, similar to this post. This is not an affiliate link, by the way, but Sweetwater does sell guitar strings.
GuitarAndBass.com: 9 Big Differences of Roundwound and Flatwound Strings - Listicle-style overview that's better than you might expect based on the format
r/guitar: Flatwound strings VS Roundwound strings? - archived discussion of the topic from Reddit's r/guitar
The Structure of a Guitar String - Older discussion other of guitar-string construction, covering core wire shape and material in addition to wind