In agreement, harmony, or correspondence; matching or compatible. Often used in mathematics to describe identical shapes, but also used to describe ideas or actions that align.
From Latin 'congruere' meaning 'to come together, agree,' from 'con-' (together) and 'gruere' (to fall). The mathematical sense developed in the 17th century from the general meaning of 'fitting together.'
Picture puzzle pieces that are congruent - they fit together perfectly because they 'come together' (con-) in harmony. In math class, congruent triangles are identical twins!
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