Universal markedness theory claims that certain sounds are rarer than others. Because they are rarer, they are more difficult to pronounce and acquired by children at a later stage than more common sounds. Rare sounds are marked sounds, and common sounds are ummarked.
Universal markedness theory can make many universal generalizations about the sounds of languages. The following statements are all true:
1) Short vowels are more common than long vowels. All languages have short vowels, but not all languages have long vowels. If a language has long vowels, it also has short vowels.
2) Oral vowels are more common than nasal vowels. All languages have oral vowels, but not all languages have nasal vowels. If a language has nasal vowels, it also has oral vowels.
3) Voiceless plosives are more common than voiced plosives. If a language has a voiced plosive, it also has a voiceless counterpart.
4) Short consonants are more common than long consonants. All languages have short consonants, but not all languages have long consonants. If a language has long consonants, it also has short consonants.
5) Simple onsets are more common than complex onsets. All languages have simple onsets, but not all languages have complex onsets. If a language has complex onsets, it also has simple onsets.
Universal markedness theory helps to clarify the relationship between marked and unmarked sounds. Short vowels are more common than long vowels because they are simpler to produce. A long vowel has a longer duration if the contrast is quantitative and is a diphthong or a less common vowel if the contrast is qualitative. Oral vowels are easier to articulate than nasal ones. Voiceless plosives require less articulatory effort than voiced ones because the glottis does not vibrate in their production. Short consonants have a shorter duration than long ones and are thus easier to pronounce. Simple onsets have only one segment in contrast to complex ones which have two or more. It is clear that marked sounds are rarer because they require more articulatory effort.