A caliper made by Somet Czechoslovakia is made of hardened stainless steel. It is built with a measuring head with two measurement options:  on one side of the " head" is the external measuring, on the other side of the head the internal measuring. The head of the ruler opens (like a mouth) by pressing a clip located under the opening part. The tool being external measured is inserted between the two parts of the "mouth", for internal measurement the measuring part is located into the part that is measured. Next to the static part of the head there is a ruler of 16 cm or 7 inches long. On both sides of the ruler  connected to the opening part there is a secondary scale with which the measurement is carried out with the possibility of more accurate  results than a tenth of cm. On the back of the caliper is a table with which you can calculate the circumference of circles from the diameter measurement..