-ary, -ery or -ory
Three endings, one murmur. A related word that puts the stress on that syllable is the clearest way to hear which vowel belongs there.
The rule
-ary, -ery and -ory all flatten to the same unstressed sound. Saying a relative that puts the stress on that syllable often gives the vowel back.
Library has librarian, secretary has secretarial, history has historian, territory has territorial, and mystery has mysterious. In each relative the stress lands on the syllable in doubt, and the vowel can be heard.
-ery is often a noun for a place, a quality or a collection, made from a shorter word: bakery, bravery, machinery, scenery.
Where it stops holding
A pair worth knowing: stationary means not moving and stationery means paper and envelopes. They sound the same and differ only in this vowel, and the E of envelope is one way to keep them apart.
Where no relative moves the stress, as in salary and boundary, the ending is learned with the word.
Background
Using a relative to recover a vowel speech hides is reasoning from meaning, which the research on word structure finds worth teaching. That research is on how people learn to spell.
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