Here is the rule, stated plainly: when an American English adjective ending in -ary becomes an adverb ending in -arily, the main stress moves to the -ar- syllable. The adjective necessary is NEC-es-sar-y /ˈnɛsəˌsɛri/, but the adverb necessarily is nec-es-SAR-i-ly /ˌnɛsəˈsɛrəli/. The stress does not stay where the adjective had it; it jumps forward, and everything after the -ar- beat collapses into weak syllables.
Most learners build these adverbs the way English usually builds adverbs: take the adjective, keep its stress, add -ly. That works for quick, quickly and happy, happily. It fails for the -arily family, and the result is a five-syllable word with four unstressed syllables in a row, something English rhythm simply refuses to do. Listeners hear a word that starts strong and then falls apart.
The rule in formal terms
- Adjective in -ary: main stress early in the word, and in American English the -ar- syllable keeps a secondary stress with a full /ɛ/ vowel: necessary /ˈnɛsəˌsɛri/, temporary /ˈtɛmpəˌrɛri/.
- Adverb in -arily: that secondary stress is promoted to main stress: /ˌnɛsəˈsɛrəli/, /ˌtɛmpəˈrɛrəli/. The old main stress weakens to secondary.
- After the stressed -ar-, the ending is always the same weak tail: /-ərəli/ → -er-uh-lee, quick and quiet.
If you already know how the -ary adjectives work, this is the natural next step; if not, start with our guide to the -ary, -ory, -ery endings and come back.
Why the stress moves
English resists long runs of unstressed syllables. Rhythm wants a beat roughly every two or three syllables. Keep the adjective stress in necessarily and you get NEC-uh-sehr-uh-lee: one beat followed by four weak syllables. That string is so awkward that even native speakers who try it end up compressing the word. American English solves the problem by promoting the secondary stress that was already sitting on -ar-: nec-es-SAR-i-ly gives a beat, two weak syllables, a beat, two weak syllables. Perfect alternation.
This is the same force behind pairs like photograph and photography: suffixes reorganize the rhythm of the whole word. The -arily shift is simply the version of that law that applies to adverbs.
The ten adverbs worth mastering
| Adjective | Adverb | American IPA | Say it as |
|---|---|---|---|
| necessary | necessarily | /ˌnɛsəˈsɛrəli/ | nec-uh-SEHR-uh-lee |
| temporary | temporarily | /ˌtɛmpəˈrɛrəli/ | tem-puh-REHR-uh-lee |
| primary | primarily | /praɪˈmɛrəli/ | pry-MEHR-uh-lee |
| ordinary | ordinarily | /ˌɔrdənˈɛrəli/ | or-dun-EHR-uh-lee |
| voluntary | voluntarily | /ˌvɑlənˈtɛrəli/ | vah-lun-TEHR-uh-lee |
| momentary | momentarily | /ˌmoʊmənˈtɛrəli/ | moh-mun-TEHR-uh-lee |
| arbitrary | arbitrarily | /ˌɑrbəˈtrɛrəli/ | ar-buh-TREHR-uh-lee |
| military | militarily | /ˌmɪləˈtɛrəli/ | mil-uh-TEHR-uh-lee |
| customary | customarily | /ˌkʌstəˈmɛrəli/ | kus-tuh-MEHR-uh-lee |
| extraordinary | extraordinarily | /ɪkˌstrɔrdənˈɛrəli/ | ek-stror-dun-EHR-uh-lee |
Notice what stays constant: the stressed syllable always carries a full, clear /ɛr/ (the vowel of berry), and the tail is always the same murmured -uh-lee. Once you own that tail, every word in the table is the same job with a different front half.
Note: primarily has one less syllable than you think
Primary is three syllables, so learners expect primarily to keep the PRI- stress. In American English it is four syllables with the beat on -mar-: /praɪˈmɛrəli/. The same goes for contrary, contrarily /kənˈtrɛrəli/. Short base, same rule: the -ar- takes the beat.
What about British English?
You will hear both patterns in Britain. Traditional RP kept the adjective stress and squeezed the word, so necessarily could come out as /ˈnɛsəsrəli/, four syllables. Modern British speakers increasingly use the American pattern. For this site we teach the American form: stress the -ar-, keep all five syllables, let the tail go weak. It is the pattern you will hear in US media, and it is easier to say.
The two-step drill
Step 1: find the beat. Say only the stressed syllable loudly and clearly: SEHR. REHR. MEHR. Feel the jaw drop on /ɛ/.
Step 2: attach the tail. Add the weak ending: SEHR-uh-lee. Then build backward: uh-SEHR-uh-lee, nec-uh-SEHR-uh-lee. Building from the stressed syllable outward stops the old adjective stress from sneaking back in.
Practice words
Drill sentences
- That is not necessarily true.
- The bridge is temporarily closed, primarily for repairs.
- Ordinarily we do not accept late applications.
- He resigned voluntarily, not under pressure.
- The lights dimmed momentarily, then came back.
Say each sentence three times, tapping the table on every -SEHR-, -REHR-, -MEHR- beat. Then keep training long-word rhythm with our word practice exercises and full sentences in sentence practice.