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Curl is generally considered to be the tendency of paper or paperboard to curl or roll up in the cross machine direction, usually in response to changes in relative humidity (RH). This is often referred to as "CD curl". Machine direction curl (MD) is often called "roll set curl".

Curl tendency can be measured using a variety of sample sizes and sample orientations. We measure curl at various relative humidities, but the normal tests are performed at 50% RH and 20% RH. Our normal sample size is 1inch x 6 inches, with the long dimension in the cross direction for CD curl. We typically use multiple samples and place the samples on a flat surface.