Pen, writing unit and a device for holding a writing unit

The invention relates to a pen comprising a first casing and a writing unit, which is inserted into said first casing. According to the invention, the pen comprises the following: a first stop, which is coupled to the writing unit and acts in a peripheral direction about the longitudinal axis (L) of said writing unit; a second stop, which is coupled to the first casing, acts in a peripheral direction about the longitudinal axis and interacts with the first stop in a predetermined first rotational position of the writing unit with respect to the first casing, the first and second stops abutting each other in the first predetermined rotational position; a third stop, which is coupled to the writing unit and acts in the longitudinal direction, in addition to a fourth stop, which is coupled to the first casing, acts in the longitudinal direction and interacts with the third stop. Said third stop is configured as a guide device for the fourth stop. The fourth stop can be displaced in relation to the first casing along said guide device during the rotation of the writing unit, until the predetermined first rotational position has been attained.