The popular general encyclopedias are most impressive by their size and number of volumes. The famous adult encyclopaedia are remembered as the ABC's - that is Americana, Britannica and Collier's. These encyclopedias are very useful for comprehensive coverage of various topics. For example, if one wants to know all about aeroplanes or butterflies, a good common encyclopedia would be a good starting point. For any American topic, say some historical aspect or geography of some place, the Encyclopedia Americana would be particularly useful.