In English, there're some words that have two spellings and have exactly the same meaning. The words that I want to ask are not those different spellings between British and American English.
These words are as follows:
1. pertinence/pertinency
http://www.merriam-webster.com/dictionary/pertinency
2. subsistence/subsistency
http://www.thefreedictionary.com/subsistency
I just listed four words above. Is there any difference between these words? Is it OK to spell N-encE into N-encY(because almost lots of words have both spelling)?
Thank for your help!
Generally  x-ce is specific,  x-cy  broad, abstract.
The squirrel's winter subsistence consists of the hoards gathered in the autumn.
The squirrel's winter subsistency depends on a range of adaptative behaviors.
(Though most times the 2 forms would be interchangeable.)
One counter-example might be consistence versus consistency. The consistence of uncured concrete is cement, sand, aggregate, and water. Its consistency is dry, type-of-slump, watery, etc.