A mora is an abstract unit of sound weight used in linguistics to measure how “heavy” or “light” a syllable is. Think of it as a building block smaller than a syllable but larger than a single sound. A light syllable contains one mora, a heavy syllable contains two, and some languages even allow superheavy syllables with three. This concept matters because many languages use moraic weight to determine things like word stress, poetic rhythm, and the timing of speech.
How Moras Map to Sounds
Not every sound in a syllable contributes a mora. The core rule is straightforward: short vowels carry one mora, while long vowels and diphthongs (two-vowel combinations like “oi” or “ai”) carry two. The consonant at the end of a syllable, called a coda, may or may not add a mora depending on the language. The consonant at the beginning of a syllable, called an onset, almost never counts.
This gives you a simple formula. A syllable like “ba” (consonant plus short vowel) has one mora and is light. A syllable like “baa” (consonant plus long vowel) has two moras and is heavy. A syllable like “bat” (consonant plus short vowel plus final consonant) could be either light or heavy depending on the specific language’s rules. English, for instance, treats closed syllables like “bat” as heavy, while some other languages do not.
A small number of languages break the usual pattern by allowing onsets to carry moraic weight, but this is rare enough that most phonological models treat it as the exception rather than the rule.
Japanese: The Textbook Example
Japanese is the language most often used to explain moras because its entire sound system is organized around them. In Japanese, words are measured not by syllables but by moras, and speakers have a strong intuitive sense of how many moras a word contains.
The word いぬ (inu, meaning “dog”) has two moras: い (i) and ぬ (nu). The word すずめ (suzume, meaning “sparrow”) has three: す (su), ず (zu), and め (me). So far, moras and syllables line up neatly. But Japanese also counts certain sounds as their own separate moras even when English speakers would lump them into a single syllable. A long vowel counts as two moras. The nasal sound “n” at the end of a syllable counts as its own mora. The first half of a double consonant counts as a mora too.
Take the word きゅうり (kyuuri, meaning “cucumber”). An English speaker might hear two or three syllables, but Japanese divides it into three moras: きゅ (kyu), う (u), and り (ri). The small characters ゃ, ゅ, and ょ combine with the preceding sound into a single mora rather than standing alone. This moraic structure shapes everything from the rhythm of spoken Japanese to the rules of haiku, where the traditional “5-7-5” count refers to moras, not syllables.
Mora-Timed vs. Syllable-Timed Languages
Linguists classify languages partly by what controls their rhythmic timing. In a syllable-timed language like Spanish, each syllable takes roughly the same amount of time regardless of its internal structure. In a mora-timed language like Japanese or Finnish, it’s the mora that serves as the rhythmic heartbeat. Each mora occupies roughly the same duration, so a heavy (two-mora) syllable takes about twice as long to say as a light (one-mora) syllable.
Acoustic studies of Finnish, a mora-timed language, confirm this. When researchers measured disyllabic words, adding moras to either vowels or consonants consistently increased the total duration of the utterance in predictable increments. The timing system adjusts both vowel and consonant durations to fill out or limit the intervals between vowels. In syllable-timed languages, by contrast, timing works more directly from vowel to vowel without that fine-grained moraic adjustment.
Moras in Classical Poetry
The concept of the mora long predates modern linguistics. Ancient Greek and Latin poets built their entire metrical systems around syllable weight, and the mora was the underlying unit that made those systems work. Greek lyric meter, for example, was constructed entirely from patterns of heavy and light syllables rather than from word stress or pitch accent.
Every metrical position in Greek verse corresponds to a “moraic trochee,” which is either one heavy syllable (two moras) or two light syllables (one mora each). A dactylic rhythm creates a pattern of stress clashes, while a trochaic rhythm creates evenly spaced lapses. The key insight is that a long syllable and two short syllables were treated as metrically equivalent because both total two moras. This is why the term “mora” itself comes from the Latin word for “delay”: it originally described the unit of time that a short syllable occupied in verse.
Why Moras Matter Beyond Poetry
In many languages, moraic weight determines where stress falls in a word. Languages with weight-sensitive stress systems place emphasis on heavy syllables over light ones. If you’ve ever noticed that English words like “agénda” stress the second syllable while “cínema” stresses the first, syllable weight is part of the explanation.
Moras also govern minimum word size in some languages. Many languages require every content word to be at least two moras long. This is called the “minimal word constraint,” and it explains why certain short words get lengthened or why some very brief roots always appear with additional sounds attached.
For language learners, understanding moras can clarify pronunciation challenges. If you’re learning Japanese, knowing that a long vowel and a short vowel differ by exactly one mora helps you produce the correct word length rather than accidentally saying a different word. The difference between おばさん (obasan, “aunt,” three moras for oba) and おばあさん (obaasan, “grandmother,” four moras for obaa) is a single mora of vowel length, and native speakers hear that distinction clearly.