From Khuddaka Nikaya's The Questions of King Milinda (translation mine):
‘Venerable Nagasena, what is the characteristic mark of
vitakka?’‘The characteristic mark of
vitakkais "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.’
‘Venerable Nagasena, what is the characteristic mark of
vicāra?’‘The characteristic mark of
vicārais 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.’