Crossword Genius

Case of correspondence between London and Washington? (7,7)

Ross

I believe the answer is:

capital letters

I'm a little stuck... Click here to teach me more about this clue!

'case of correspondence between' is the definition.
I know nothing about this answer so I can't tell whether it can be defined by this definition.

'london and washington?' is the wordplay.
'london' becomes 'capital' (I've seen this before).
'and' means one lot of letters go next to another.
'washington?' becomes 'letters' (I am not sure about this - if you are sure you should give a lot more credence to this answer).
'capital'+'letters'='CAPITAL LETTERS'

Can you help me to learn more?

(Other definitions for capital letters that I've seen before include "As for example" , "blocks" , "big characters" , "Athenian landlords?" .)

I've seen this clue in The Telegraph.
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