Adding a stroke makes a consonant aspirated (with a puff of air). Doubling it makes it tense (no air, tight throat).
| plain | aspirated | tense |
|---|---|---|
| ㄱ g | ㅋ k | ㄲ kk |
| ㄷ d | ㅌ t | ㄸ tt |
| ㅂ b | ㅍ p | ㅃ pp |
| ㅈ j | ㅊ ch | ㅉ jj |
| ㅅ s | ㅆ ss |
Hold a hand in front of your mouth: 카 should give a strong puff, 까 almost none (like the k in "skate").
These three-way contrasts change meaning: 불 (fire), 풀 (grass), 뿔 (horn).