The noun referring to something issued or paid at intervals is spelled installment in the U.S. Outside the U.S., it’s spelled with one l—instalment.
Canada is the only English-speaking country outside the U.S. where installment is common; it appears in 21st-century Canadian books and news publications about twice for every three instances of instalment.
Both spellings are as old as the word itself, which originated in the 16th century, and the UK or British spelling was not permanently settled until the 18th century. That spelling was also preferred in American English until around 1920, when the the double-l spelling gained ascendancy.
Since then, instalment has grown less and less common in American usage, and today it is scarce in U.S. publications.
Other words have developed similarly. Fulfilment and enrolment are so spelled outside North America, while fulfillment and enrollment are preferred in the U.S. (Canadians don’t seem to have consensus on these).
These cases are slightly different from instalment vs. installment, though, as the distinction is borne out in the root words; enroll and fulfill are the spellings in the U.S., while enrol and fulfil are preferred everywhere else. Install is spelled install everywhere.
Examples of Fulfill Being Used in a Sentence in the UK and America
Another area that may come under its microscope: installment lenders. [Wall Street Journal]
The middle installment of Green Day’s ambitious garage-rock trilogy is the troubled part of the story. [Newsday]
The second installment of property taxes are due February 1. [Orange County Breeze]
The third and final instalment of Pope Benedict’s biography of Jesus is published on Wednesday. [BBC]
The deal … opens that way for the next instalment of financial aid in December. [Globe and Mail]
[A] very lean budget was adopted by Greek officials, securing the indebted nation’s next bailout instalment of 31.5 billion euros. [Stuff.co.nz]
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