How does Korean distinguish simple consonants from aspirated consonants?

In the Korean language, the distinction between consonants is not based on voicing, as voicing is automatic. Instead, Korean differentiates between three types of obstruents: simple (lax), heavily aspirated, and reinforced (tense) consonants.1

• Simple (Lax) Consonants: These are the base forms, such as p, t, k, and ch. In the case of the sibilant s, it is considered a lax consonant that is somewhat aspirated.1
• Heavily Aspirated Consonants: These are produced with a strong burst of air. They are represented in romanization with an apostrophe, such as p’, t’, k’, and ch’.1
• Reinforced (Tense) Consonants: These are produced with increased muscular tension and are written as geminates (duplicate letters), such as pp, tt, kk, and tch. The tense sibilant ss also falls into this category.1

Historically, these distinctions evolved from the reduction of earlier consonant clusters. For example, reinforced consonants likely emerged from the reduction of clusters like st or pt, while heavily aspirated consonants often trace back to earlier clusters involving h or k.1