Claims triangulation is the technique of arranging an insurer's claims data in a table in which each row is a period of origin, and each column is a stage of development. Each cell holds the cumulative amount paid or incurred for that origin period at that stage. Because recent periods have had less time to develop, the filled cells form a triangle, and the task of claims reserving is to complete the missing lower half.
Reading down the columns shows how claims typically grow from one stage to the next. The ratios between successive columns give a loss development factor for each stage, which the chain ladder method applies to newer periods to project their ultimate cost. The difference between that projection and the amount reported so far is the estimate of incurred but not reported (IBNR) claims. Triangles also show how the claims reserve has run off.