Inch
| Inch | |
|---|---|
| Unit system | imperial/US units |
| Unit of | length |
| Symbol | in |
| 1 in in ... | ... is equal to ... |
| imperial/US units |
1/36 yd 1/12 ft |
| metric (SI) units | 25.4 mm |
An inch (abbreviation: in or ″) is a unit of length in the imperial and United States customary systems of measurement now formally equal to 1⁄36 yard but usually understood as 1⁄12 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 2.54 cm.
The English word "inch" (Old English: ynce) was an early borrowing from Latin uncia ("one-twelfth; Roman inch; Roman ounce") not present in other Germanic languages. The vowel change from Latin /u/ to English /ɪ/ is known as umlaut. The consonant change from the Latin /k/ to English /tʃ/ or /ʃ/ is palatalisation. Both were features of Old English phonology. "Inch" is cognate with "ounce" (Old English: ynse), whose separate pronunciation and spelling reflect its reborrowing in Middle English from Anglo-Norman unce and ounce.
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