Musicianship is essential for any Bard skill. It is required to use any of the other bard-related skills (Provocation, Peacemaking, Discordance). It is activated through the use of various instruments.
While it does very little on its own aside from producing soothing music (or a fat-finger cacophony), Musicianship’s skill level is a key factor in determining the success and effectiveness of Discordance, Peacemaking, and Provocation.
Formula for all Bard skills
Success = Relevant Bard Skill - Barding Difficulty + 30 Base Chance + 10 for each of the other two instrument skills if they are at GM +/- 10 if you have the correct slayer flute or not.
This gives a maximum chance at equal skill vs barding difficulty of 30% + 2 X 10% + 10% = 60%
See our Bard Difficulty Calculator (experimental calculator)
It is possible to actively and passively train Musicianship.
For each bard skill that is 100, the distance range requirement is increased to a slightly higher range as a perk to grandmasters of the class.