The general characteristics of the producing oil and gas fields in that portion of the Rocky Mountain region, comprising Wyoming, Colorado, and northwestern New Mexico, are briefly discussed. This area is naturally divisible into three distinct provinces: an intensely folded, broken, and overthrust area in the west; a central area prominently folded into large and small anticlines and synclines; and an eastern area in which the folding is much less pronounced and the structure approaches the Mid-Continent type. Rules for determining productive versus non-productive structural features in advance of drilling are formulated. In actual practice it is not always possible to abide by all of these rules, as some of the conditions are not everywhere determinable in advance of drilling. An otherwise favorable structural feature is not lightly passed even if all the conditions prescribed in these rules are not inherent to it, because ideally favorable structural features are becoming scarce.