Serves as a ruler that controls the length of tail by stopping the tail tube polymerization and is probably released from the tail shaft during infection to facilitate DNA translocation into the host cell. Assembles into a multimeric linear form probably arranged as a coil of alpha-helices and stabilized by the covering tail assembly proteins. Its C-terminus fixes the tail tip complex, thereby forming the tail assembly initiator complex. Tail tube proteins polymerize around the tape measure protein, displacing the tail assembly proteins. When the tail reaches the length specified by the tape measure protein, it stops and becomes capped by the tail terminator protein. Interacts with the tail initiator complex presumably through its C-terminus domain. Interacts with the tail assembly proteins. Belongs to the Lambdavirus tape measure protein family.