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Why are nylon and polyester considered synthetic fibres?

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Diagram titled Spinning jenny showing an operator using a hand wheel to rotate a cord-driven roller, drawing and twisting fibers from multiple clamped rovings onto tilted spindles; numbered callouts trace the process from roving to thread. — textile An annotated diagram showing how an operator uses a hand wheel, rollers, and spindles on a spinning jenny to draw and twist unspun fiber into thread.

Nylon and polyester are classified as synthetic fibres because they are composed of polymers that do not occur in nature. Unlike natural fibres—such as cotton, silk, or wool—or regenerated fibres like rayon, which are derived from naturally occurring cellulose, synthetic fibres are produced entirely through chemical synthesis in a laboratory or industrial plant.1

These materials are typically manufactured from by-products of petroleum or natural gas. For example, nylon is created by synthesizing polyamides from chemical compounds, while polyester is built from repeating units linked by ester groups, often through a condensation reaction between an alcohol and a carboxylic acid. Because they are engineered from the molecular level up, these fibres can be mass-produced to meet specific requirements for strength, elasticity, and resistance to heat or chemicals.123