From the Franklin Mint Blog:
Aragonés became famous for his wordless "drawn-out dramas" or "marginals" which were inserted into the margins and between panels of the magazine. The drawings are both horizontal and vertical, and occasionally extend around corners. Prior to Aragonés' arrival at MAD, the magazine had sometimes filled its margins with text jokes under the catch-all heading "Marginal Thinking." According to Aragonés the staff of MAD enjoyed his marginals, but expected him to only last one or two issues. They did not expect him to be able to maintain the steady stream of small cartoons needed for each issue. However, Aragonés has provided marginals for every issue of Mad since 1963 except one (his contributions to that issue were lost by the Post Office).