The Low C bass clarinet manufacturer tells you that the bass clarinet is a transposing instrument. In the 19th century, the clarinet's notation was followed, using the treble clef, but the actual pitch was 1 major ninth lower than the notation. The Low C bass clarinet manufacturer tells you that the treble clef is used because the bass clarinet is performed by the clarinetist, and using the same clef greatly reduces the inconvenience of reading notation and makes it easier for the player to switch instruments.

The Low C bass clarinet manufacturer tells you that by the 20th century, composers would switch to using the bass clef, so that the actual pitch was 1 major second lower than the notation, and then switched back to using the treble clef ( But there are also nine or two degrees apart. Parts are still the original treble clef.
The Low C bass clarinet manufacturer tells you that the bass clarinet is an instrument of the clarinet family. Like the more common soprano B♭ clarinet, usually represented by a B ♭ (indicating that this is a transposing instrument where the C written above sounds like a B♭), but it plays a note an octave below the soprano B♭ clarinet.
The above is the Low C bass clarinet manufacturer's introduction and analysis of the notation of the bass clarinet, hope it helps you.