Pen and adapter for detachably attaching pen to wrist strap

A pen, comprising:

• a pen body having a rectangular shape cross-section;
• and
• a cartridge seating portion provided in the pen body, wherein a cartridge is detachably seated in the cartridge seating portion,
• wherein the cartridge seating portion extends diagonally across the rectangular cross-section between two non-adjacent vertices of the pen body,
• wherein the pen body includes through-holes respectively formed through left and right sides of the pen body with respect to the cartridge seating portion,
• wherein the cartridge seating portion defines a tubular passage into which the cartridge is insertable,
• wherein the pen further comprises a cartridge removal portion for removing the cartridge from the cartridge seating portion,
• wherein the cartridge removal portion comprises a cartridge rear-end insertion hole that:
• is formed through a rear end of the cartridge seating portion such that a rear end of a cartridge in use is partially exposed;
• is formed to have a size into which a rear end of an external replacement cartridge is insertable;
• and
• guides, when the rear end of the replacement cartridge is inserted, the cartridge in use to be withdrawn through a distal end of the cartridge seating portion, and
• wherein a center of the cartridge rear-end insertion hole is offset to one side from a center of the cartridge seating portion.