Abstract: A number board apparatus for teaching beginning set and number theory to young or educationally handicapped children and including a carrying tray having a generally rectangular base with an upright sidewall rising from each of its edges, a plurality of work blocks freely slideable on a top surface of the base until contacting a sidewall or another block, each work block having indentations in the form of a numeral and/or a rectangle which are receptive to removable numeral or rectangular unit marker pegs, respectively. Several optional members such as covering boards and specialized work blocks may also be used with the number board apparatus.