PREFACE Knowledge of the various aspects and elements of city planning does not advance logically and at an even pace over the whole field. Rather it moves irregularly, and facts and theories are brought down to details where they happen to be most needed at the moment. In this way - at least provisionally, for practical use definite city planning con- ceptions as to parks, zoning, public acquisition of land, and immediate traffic relief have been already set up and accepted. At the moment there is forced upon us a very significant problem, that of building lines. That it has not yet been fully solved is made all the more evident by other recent gains in clarity of thought. It is not essentially new. Theauthors ofModelLawsfor Planning Cities, Counties, and States, and many other previous writers, recognized that in this matter we were proceeding without any really sufficient knowledge of present diverse and sporadic experience, and thus without a sound basis for constructive theory. This situation is caused by the interval of time which normally exists between the acceptance by a community of a plan which involves the public acquisition of land, and the actual complete acquisition of this land. We are unable to assure the plans being carried out, because, during this interval, its eventual realization may be hampered, intention- ally or accidentally, by private constructions for which their owners are legally entitled to compensation. It is to be noted that this time interval between planned purpose and land acquisition is inevitable, indeed it is a part of the plan, since no city can or should waste the public funds and the interim land use by immediate acquisition of all the land required for ultimate public development. Some solution of the problem is imperative, and apparently it is to be sought by considering what rights in land the city may desirably take, short of taking the whole plexus of rights which we call land ownership...