Caution needs to be exercised, however, to ensure that both sources of instruction truly are necessary for the to-be-learned information to be intelligible.In situations where a source of textual instruction, or a source of graphical instruction alone provides full intelligibility then only one source of instruction should be used (either the textual or the graphical), and the other source, which is redundant, should be removed completely from the instructional materials. In these contexts a single source of instruction returns higher levels of learning than either an integrated format (text integrated into the graphic), or a dual format (both text and graphic presented in parallel).Cognitive load theory explains this result by focussing on the levels of cognitive load imposed upon the learner who needs to process the varying instructional materials.Attending to both textual and graphical sources of instruction requires more mental resources than attending to a single source. Attending to both textual and graphical sources of instruction, therefore, results in a reduced portion of working memory being available for the process of learning.Maps, whether their purpose is to locate countries (an atlas), indicate the steepness of terrain (a topographic map) or to show the way to get from A to B (a street directory) are examples of graphically based sources of instruction that are fully self contained. Provided the user has the skills to read and interpret a map, then there is no need for any associated body of textual information.Similarly, many instances of textual instruction have no need for graphics. Arguments of litigation, analysis of history and the use of a dictionary or thesaurus are fully intelligible in a text-only format. The use of graphics in these situations actually reduces the level of learning that results from the use of these documents.