c1

C1 is a designation that appears in multiple disciplines to denote a first form, level, or component within a broader system. Because its meaning depends on context, the term is used across anatomy, linguistics, mathematics, music, and consumer products, among others.

In anatomy, C1 refers to the first cervical vertebra, also known as the atlas. The atlas supports the skull and forms the atlanto-occipital joint with the occipital bone, enabling the nodding motion of the head. It articulates with the second cervical vertebra, the axis (C2), via the atlantoaxial joint, allowing some degree of rotational movement. The C1 vertebra is a critical structure in spinal biomechanics and can be involved in trauma and congenital anomalies.

In language education, C1 denotes a level within the Common European Framework of Reference for Languages (CEFR). A C1 user is considered a proficient user who can understand demanding, long texts, recognize implicit meaning, and express ideas fluently and spontaneously. They can use language flexibly for social, academic, and professional purposes and produce clear, well-structured, detailed text.

In mathematics, C1 refers to a class of functions with continuous first derivatives on a given domain, denoted as C1 or C^1. Such functions are differentiable, and their derivatives are continuous, a property important in analysis, differential equations, and differential geometry.

In music and acoustics, C1 identifies the note C in the first octave above C0, with a frequency of about 32.70 Hz. In scientific pitch notation, C1 is used to label that specific pitch, commonly played on musical instruments and referenced in tuning contexts.

C1 is also the model name of the Citroën C1, a city car introduced in 2005 (developed alongside the Peugeot 107 and Toyota Aygo). The C1 line has included multiple generations and variants, reflecting its role as a compact urban vehicle.

Overall, C1 functions as a versatile shorthand across fields, its precise meaning determined by the surrounding domain.