Sentence:  The distinction between <WSD>custom</WSD> and usage it that usage is a fact and <WSD>custom</WSD> is a law. There can be usage without <WSD>custom</WSD>, but not <WSD>custom</WSD>  without usage. 
Meaning of the word marked with <WSD> tags:  Long-established practice, considered as unwritten law, and resting for authority on long consent; usage. See Usage, and Prescription.