It is not the gay coat that makes the gentleman. 
Proverbs on a similar topic
- All that glitters is not gold.
- Still waters run deep.
- There is truth in wine.
- None so deaf as those who will not hear.
- Many a true word is spoken in jest.
- A fair face may hide a foul heart.
- Iron hand in a velvet glove.
- Barking dogs seldom bite.
- Lies have short legs.
- Truth is mighty and will prevail.
- Appearances are deceptive.
- A honey tongue, a heart of gall.
- The cowl does not make the monk.
- The biter bit.
- The face is no index to the heart.
- A liar is not believed when he speaks the truth.
- All are not thieves that dogs bark at.
- Children and fools tell the truth.
- A little body often harbours a great soul.
- All are not hunters that blow the horn.
- When candles are out, all cats are grey.
- All are good lasses, but whence come the bad wives?
- A traveller may lie with authority.
- Great boast and small roast.
- You cannot hide an eel in a sack.
- Liars have need of good memories.
- Truth has a scratched face.
- Fair without, false within.
Related proverbs
Similar proverbs in another language
Latin
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In vili veste nemo tractatur honeste.
No one is treated decently in cheap clothes.
French
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Il faut traiter les gens selon leur mérite.
You have to treat people according to their merit.
German
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Man empfängt den Mann nach seinem Gewande und entläßt ihn nach seinem Verstande.
A man is received according to his garment and dismissed according to his mind.
Czech
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Dle šatu vítají, dle rozumu provázejí.
According to dress they welcome, according to reason they accompany.