Prepositions begin with concrete relationships in space, time, and direction, so a speaker can imagine one thing being inside, on, under, before, after, or toward another. This physical picture is the foundation of the form, even when the same preposition later appears in fixed expressions. Prepositions and Common Prepositions provide the basic system that these idiomatic uses extend.

In many common expressions, a preposition keeps its form but loses its literal meaning and becomes part of a fixed phrase. The meaning of the whole expression is then idiomatic and cannot be predicted exactly from the separate words. These patterns often need to be learned as complete phrases, especially alongside Prepositional Phrases.

Word or Phrase	Definition	Example
in time	Early enough to do something before a deadline or problem.	The train arrived just in time, so we were not late
on purpose	Intentionally and not by accident.	He left the note on purpose, so everyone would see it
by chance	Without planning, and because of luck.	We met by chance at the station
out of luck	With no success or no remaining chance.	I was out of luck when the last ticket sold out
in trouble	In a difficult or risky situation.	She was in trouble when the report was missing
in doubt	Not certain about something.	He was in doubt until the final answer arrived
on board	As part of a plan, group, or agreement.	Everyone was on board after the manager explained the idea
by hand	Using your hands instead of a machine.	The cards were made by hand, so each one was unique
at once	Immediately or all together.	They left at once when the alarm sounded
out of hand	No longer controlled.	The argument got out of hand very quickly

Idiomatic prepositions appear most often in everyday conversation, especially in directions, emotions, decisions, mistakes, and situations of chance. Many are informal and natural in speech, although some are also acceptable in more formal writing when the expression is well established. Fixed idioms resist change, so native speakers usually prefer the expected preposition rather than a near alternative.

Region	Word or Phrase	Regional Definition	Example
British English	at weekends	This form is common in British English for recurring weekend time.	I relax at weekends, so I read and cook
American English	on weekends	This form is common in American English for recurring weekend time.	I relax on weekends, so I read and cook
General	fixed idiom	This expression keeps one stable preposition and sounds unnatural if changed.	People say on purpose, so by purpose sounds wrong
Informal	conversational use	This pattern is especially natural in everyday speech and dialogue.	We met by chance, so the story sounds friendly
Formal	established phrase	Some idioms are accepted in formal contexts when they are standard.	The plan was approved in time, so the deadline was met

Some idiomatic prepositions overlap in meaning with prepositional phrases, but their exact wording remains fixed. Learners often meet them near expressions of time, place, and direction, so Prepositions of Time, Prepositions of Place, and Prepositions of Direction help keep the literal system clear while the idiomatic system is memorized as whole units.

Word or Phrase	Definition	Example
in time	Early enough for the result to succeed.	We arrived in time for dinner
on time	At the expected or correct time.	The bus arrived on time, so we were calm
by chance	Without planning and through luck.	We spoke by chance, so the meeting felt surprising
on purpose	Done intentionally and with a goal.	She corrected the error on purpose, so the message was clear
out of place	Not suitable for the situation.	His joke felt out of place, so the room went quiet
at once	Immediately or without delay.	They responded at once, so the issue was solved
by hand	Using manual effort rather than a machine.	The sign was painted by hand, so it looked personal
in front of	At the position before something.	The taxi stopped in front of the hotel, so we got out
into	Moving toward the inside of a place.	She walked into the room, so the conversation stopped
out of	Moving from inside to outside.	He stepped out of the car, so the crowd could see him

Idiomatic prepositions sound best when they are used in short, natural phrases rather than isolated rules. In conversation, speakers choose them to express intention, luck, control, timing, and direction with a quick and familiar rhythm. In longer writing, the same expressions remain useful when they are already established and precise.

Word or Phrase	Definition	Example
on purpose	with intention	I said it on purpose, so the point was clear
by chance	without planning	We met by chance, so the day felt lucky
in time	early enough	She finished in time, so she could join us
at once	immediately	They ordered at once, so the waiter smiled
out of luck	without success	I was out of luck, so I missed the last bus
in trouble	in a difficult situation	He was in trouble, so he asked for help
on board	supporting a plan	The team was on board, so the idea moved forward
by hand	manually	The letter was written by hand, so it felt personal
out of hand	no longer controlled	The debate got out of hand, so the teacher stepped in
at the wheel	driving or in control	She was at the wheel, so everyone trusted her