From Khuddaka Nikaya's The Questions of King Milinda (translation mine):

>‘Venerable Nagasena, what is the characteristic mark of `vitakka`?’
>
>‘The characteristic mark of `vitakka` is "placing" (`Appanā`), Your Majesty.’  
>  
>‘Give me an example.’  
>  
‘For example, Your Majesty, like a carpenter takes well-finished boards of wood and places (`appeti`) them in position.’    
>  
>[Mil 3.3.13](https://suttacentral.net/mil3.3.13/en/tw_rhysdavids)

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>‘Venerable Nagasena, what is the characteristic mark of `vicāra`?’
>
>‘The characteristic mark of `vicāra` is pondering/mulling over (`anumajjana`), Your Majesty.’
>
>‘Give me an example.’
>
‘For example, Your Majesty, if you strike on a copper vessel, it then rings for a while, it responds with a sound. Here, Your Majesty, the strike is a simile for `vitakka` and the reverberation is a simile for `vicāra`.’

><a href="https://suttacentral.net/mil3.3.14/en/tw_rhysdavids">Mil 3.3.14</a>