The answer is human hair. Milady states that in today's market, human hair has become the most popular choice for hair replacement systems.
The book explains why: combining human hair with newer base designs produces a natural-looking hair replacement system that's virtually undetectable. That's a different question from what's used for theatrical or fashion wigs, where blended or mixed hair is actually the more common choice — a distinction tested in a related question on this same page.
Milady Standard Barbering, 6th ed., Ch. 15 (Men's Hair Replacement), p. 499: "In today's market, human hair has become the most popular choice when it comes to hair replacement."
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More real Hair Science questions with sourced answers: What's Called Knotting in Hair Replacement, What Are Ready-to-Wear Wigs Made Of, What Indicates the Strength of Hair's Cortex, What Does Reduced Melanin Production Cause, and 28 more on the Hair Science practice questions page.
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