This is the Harborth graph, the simplest known planar graph in which all the edges are the same length and every vertex has exactly four neighbours. pic.twitter.com/cvwK6W2ymD
The plus sign comes from the abbreviation of the Latin word "et" meaning "and". There were recorded more than 100 abbreviations for "et" before Johann Widman put + in print for the first time in 1489. pic.twitter.com/tX5oc9jQn3