The Marlinspike or Marlin Spike is a tool designed for use in marine and sailing situations. The tool is made of stainless steel with a smooth tapered shaft and an eye near the tip to aid in sewing, it is used for untying knots and splicing rope or sewing tough materials, such as sail cloth/canvas.

When rope knots become tight due to tension, and are possibly also wet, this tool is used by inserting the tip into the knot and prying the knot loose until it can be untied. Since the shaft of the tool is smooth it will not cut or damage the rope.
It can also be used to add a smaller rope to an already existing knot by splicing it into the knot using the eye of the tool.
Simple sewing jobs on a sail can also be accomplished using the eye.

Victorinox

Victorinox 93mm Marlinspike Tool

93mm Version

This tool is a descendant of the similar Victorinox tool, the sacking-needle found on much earlier models.
The 93mm marlin spike appears a little more robust than the large sacking needle, and has very smooth curvature.
The point of the sacking-needle is noticeably sharper (needle-sharp), than the rounded point of the marlinspike.