Q:Information:  - A nail , as a unit of cloth measurement , is generally a sixteenth of a yard or 21  4 inches ( 5.715 cm ) . The nail was apparently named after the practice of hammering brass nails into the counter at shops where cloth was sold . On the other hand , R D Connor , in The weights and measures of England ( p 84 ) states that the nail was the 16th part of a Roman foot , i.e. , digitus or finger , although he provides no reference to support this . Zupko 's A dictionary of weights and measures for the British Isles ( p 256 ) states that the nail was originally the distance from the thumbnail to the joint at the base of the thumb , or alternately , from the end of the middle finger to the second joint . An archaic usage of the term nail is as a sixteenth of a ( long ) hundredweight for mass , or 1 clove of 7 pound avoirdupois ( 3.175 kg ) .  - In geometric measurements, length is the most extended dimension of an object. In the International System of Quantities, length is any quantity with dimension distance. In other contexts "length" is the measured dimension of an object. For example, it is possible to cut a length of a wire which is shorter than wire thickness.  - An inch (abbreviation: in or ) is a unit of length in the imperial and United States customary systems of measurement now formally equal to yard but usually understood as of a foot. Derived from the Roman uncia ("twelfth"), "inch" is also sometimes used to translate related units in other measurement systems, usually understood as deriving from the width of the human thumb. Traditional standards for the exact length of an inch have varied in the past, but since the adoption of the international yard during the 1950s and 1960s it has been based on the metric system and reckoned as exactly 2.54cm. Name. The English word "inch" was an early borrowing from Latin "" ("one-twelfth; Roman inch; Roman ounce") not present in other Germanic languages. The vowel change from Latin /u/ to English // is known as umlaut. The consonant change from the Latin /k/ to English /t/ or // is palatalisation. Both were features of Old English phonology. "Inch" is cognate with "ounce", whose separate pronunciation and spelling reflect its reborrowing in Middle English from Anglo-Norman "unce" and "ounce".  - A yardstick is a straightedge used to physically measure lengths of up to a yard (0.9144 metres or three feet) high. Yardsticks are flat wooden boards with markings at regular intervals.  - The yard (abbreviation: yd) is an English unit of length, in both the British imperial and US customary systems of measurement, that comprises 3 feet or 36 inches. It is by international agreement in 1959 standardized as exactly 0.9144 meters. A metal yardstick originally formed the physical standard from which all other units of length were officially derived in both English systems.    Given the paragraphs above, decide what entity has the relation 'instance of' with 'unit of length'.
A:
nail