A musician would choose to use a fretless bass for sounds without frets. They would use fretless bass to create a different sound from regular guitars.
Gibson made a fretless, but a factory fretless isn't going to have the lines. Someone took a fretted and made it fretless.
The original bass guitar, invented by Paul Tutmarc in 1935, was fretless. Frets were not added until the model was innovated by Leo Fender in 1951.
Fretless bass guitar
Your question is badly written. There are no frets on a fretless bass. Most fretless basses have markers for where the frets would be. They are either lines on the fingerboard, or dots on the side. You could count the marks to see how many fret positions there are.
Frets are marked on the bass guitar the same way as they are on the electric guitar or acoustic guitar. Metal lines run across the neck. If you're talking about a fretless bass then you shouldn't have to ask this question. It would be a big mistake to try learn the fretless bass before the standard bass. The frets are in the same positions but you should instinctively know where your fingers should be before advancing to fretless
Fretless instruments, like the fretless bass or violin, have a smooth fingerboard without frets, allowing for continuous pitch variation. This enables players to produce slides, glissandos, and microtonal nuances not possible on fretted instruments. Fretless instruments offer a more expressive and fluid playing style, but they require greater precision in intonation and technique.
During his early years (Weather Report & George Duke) he used a fretless Lobue bass.
Pino Palladino on a fretless Musicman Stingray.
A great website to purchase a bass guitar of any type is Sweetwater. The site includes 4,5,6 string guitars, fretless, acoustic, and left handed basses. If you prefer an in-store purchase then Guitar Center is the best place to find the right guitar.
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Fretless Bass Piano Warm pad Ensemble Strings Violin Syn Brass 1