Thin Edge Of The Wedge Meaning

Example : 1877 The Analyst, p. 381  But the great difficulty was the getting in the thin edge of the wedge; when once they had anything to do with price they must apply it to everything.1903 Albert Shaw The American Monthly Review of Reviews, p. 196  But at last the thin edge of the wedge of reform had been pushed in.1996, Dale Belman et al., Public Sector Employment in a Time of Transition, p. 247  the thin edge of the wedge was further widened by the fact that a socialist party [...] broke precedent [...].