The TAPPI T 551 standard specifies a method for determining the thickness of paper and Cardboard, which is tested using the soft platen method. This method compresses a paper or Cardboard sample by using a soft platen under a specified pressure, and determines its thickness by measuring the change of the sample under pressure. This standard is applicable to all kinds of paper and Cardboard products, and is widely used in quality control and production process management of paper products. The standard details sample preparation, equipment requirements, test steps, and calculation methods of results, ensuring the accuracy and Repeatability of measurement results. This method is of great significance in the paper industry and helps to evaluate the performance of paper or Cardboard, such as thickness uniformity, strength, and adaptability.