These Catechisms of the Baltimore Series are arranged on a progressive plan. No. 00 gives the Prayers and Acts to be learned before the study of the Catechisms begins:—No. 0 contains one half the questions of No. 1; No. 1 half the questions of No. 2; No. 2 one-third the questions of No. 3, and No. 4 (an Explanation of the Baltimore Catechism) furnishes much additional information with copious explanations and examples.