Also "ligne parisienne". An old unit of measurement in traditional watchmaking, prior to the metric system and directly inherited from the French pied (foot) under the Ancien Régime. One line equals 2.2558 mm, rounded up to 2.26 mm. A foot (') measured 12 inches (") of 12 lignes ('") each.
The usual abbreviation is a triple apostrophe ('") after the figure. Hence a movement can be described as having a diameter of 11'" or 11 lignes which is 24.8 mm.