Full Idea
A 'right' does not stand for a purely moral notion; it began, I suppose, by standing for a legal notion, and its usage has broadened out so as to include certain things that cannot be claimed at law, but it is not yet correlative to duty.
Clarification
'Correlative' means 'precisely corresponding'
Gist of Idea
Rights were originally legal, and broadened to include other things
Source
W. David Ross (The Right and the Good [1930], §II App I)
Book Reference
Ross,W.David: 'The Right and the Good' [OUP 1930], p.53
A Reaction
Presumably 'natural rights' are those which ought to be legal rights - or they are so obvious that there is no point in discussing legal rights until the natural rights are granted. Don't we make laws because we perceive rights?