Abstract: A thermoplastic film web is subjected to a defined shrinking in a treatment apparatus with two mirror-symmetrically arranged tentering chains. For this purpose, each chain is equipped with tentering clamps that have a variable on-center spacing or pitch in the travel direction of the tentering chains, and which additionally are position adjustable in a direction other than the travel direction. The entire clamp body or clamp elements may be position variable in a direction other than the travel direction. In operation, the pitch is first reduced in the inlet and/or heating zone of the treatment apparatus. Simultaneously, the clamping position of the individual clamps or clamping element is changed, e.g. by tilting or lifting. The clamp position change takes up extra film web length resulting from the pitch change, whereby the film web assumes throughout its width a corrugated configuration. Then, the film web is cross-stretched.