Lee Rudolph contributed the example from Yeats on soc.motss. He explains:
> The eggcorn is present in the 1953 fifth printing of the first edition of the 1933 Collected
Poems, but not in the 1953 New Edition of the 1934 Collected Plays, and has been corrected without comment in the 1983 Collected Poems: A New Edition, edited by Richard J. Finneran,
whose stated intention in the preface is “to provide accurate texts”. I don’t know if we can assume it was a printer’s error, but there it is, big as life and twice as natural, and something between 74 and 52 years old.
As horses and carriages have become rare as a means of transport, the metaphor controling or restricting their movement with the help of reins has lost its transparency. The homophone _reign_, in the sense of the exercise of power, is in the process of supplanting it.
The thoroughness of the re-interpretation in the occurrences of the _rein_ > _reign_ substitution varies. _Free reign_ is entering mainstream usage. _Reigns of power_, on the other hand, only makes sense superficially; the plural remains unaccounted for.
See also [_reign»rain_](eggcorns.lascribe.net/eng…) (as in _rain supreme_), [_rain»reign_](eggcorns.lascribe.net/eng…) (as in _right as reign_) and rein»range .
[Update: 18 October 2007, Ben Zimmer] More about free rei(g)n on OUPblog here and here.