-er and -or

-er and -or

These suffixes form agent nouns denoting either a person or a thing that performs the action denoted by the word's stem; this is sometimes a word in its own right and sometimes not, e.g. dispenser, farmer, maker, porter, sailor, suitor. In theory, -er can be added to any English verb to form an agent noun; but in practice both -er and -or forms are used, sometimes as active English suffixes and sometimes as elements borrowed with the word as a whole (e.g. doctor from Old French doctour from Latin doctor). Choice between the two suffixes is purely historical and does not have any principled distinction. Note however that some agent nouns exist in two forms, e.g. adviser and advisor (the second probably influenced by advisory).

When -er is added to verbs ending in a consonant + -y, the y is normally changed to i, as in carrier, occupier, etc. Exceptions are flyer, which is now more usual than flier, and drier, which alternates with dryer.