A late 19th century 3 inch boxwood and brass caliper rule.
The 3 inch rule is subdivided into 16th of an inch and French Lines.
The reverse has a scale in millimeters.
Length: 9 cm.
French Lines or Ligne are a French historic unit of length used prior to the metric system. The loi du 19 frimaire an VIII (Law of 10 December 1799) states that one metre is equal to exactly 443.296 French lines.
The Ligne is still used by French and Swiss watchmakers to measure the size of watch movements, and in button making and ribbon manufacture.