The Paris inch, or pouce, is an old unit of measure, that among other uses, was common for giving the measurement of lenses. The Paris inch could be subdivided into 12 ligne (Paris line), and 12 Paris inches made a Paris foot. The Paris inch and Paris foot could be abbreviated with " and ' like some other inch/foot units. It was larger than the English inch and the Vienna inch, although the Vienna inch was subdivided with a decimal, not 12 lines.

A Paris inch is equal to 27.069 millimetres. The Paris inch was also known as a pouce.

A famous example of its use is in the first Great Refractor

Great refractor

Great refractor refers to a large telescope with a lens, usually the largest refractor at an observatory with an equatorial mount. The preeminence and success of this style in observational astronomy was an era in telescope use in the 19th and early 20th century. Great refractors were large...

. It had a lens made by Joseph von Fraunhofer

Joseph von Fraunhofer

Joseph von Fraunhofer was a German optician. He is known for the discovery of the dark absorption lines known as Fraunhofer lines in the Sun's spectrum, and for making excellent optical glass and achromatic telescope objectives.-Biography:Fraunhofer was born in Straubing, Bavaria...

which was 9 Paris inches in diameter, which works out to about 24 cm or 9.6 inches (English inches) aperture achromatic lens (the largest of its day).