An easy to use flip chart showing silhouettes of 239 different sizes of O-Rings. Just match the cross section of the O-Ring you want to measure on the cross section bar chart illustrated on each page. Once you determine the O-Ring cross section, go to the page which illustrates those cross section O-Rings. Simply place the O-Ring on the silhouette which matches the O-Ring and read the size directly from the chart. Laminated in plastic. 13" x 21".