SCIENCE AND TECHNOLOGY ENGLIAH FOR MECHANICAL ENGINEERING We have said that comparison, to be useful, must be systematic. To be systematic, a comparison or contrast must be between two or more items within a special area of interest-members, that is, of a group or class that is defined by a special interest brought to th material. Contrast, like comparison, is significant only when some common ground is recognized between the things contrasted-when they belong to the same significant class, the class depending upon the purpose of the contrastSCIENCE AND TECHNOLOGY ENGLISH FOR MECHANICAL ENGINEERING We have said that comparison, to be useful, must be systematic. To be systematic, a comparison or contrast must be between two or more items within a special area of interest-members, that is, of a group or class that is defined by a special interest brought to the material. Contrast, like comparison, is significant only when some common ground is recognized between the things contrasted-when they belong to the same significant class, the class depending upon the purpose of the contrast