T-21 has the traditional USAF-1951 spiral layout with improved labeling meeting the requirements as specified in MIL-STD-150A. The chart has Direct Frequency Labeling in C/MM adjacent to their features. Thus no cross reference documentation is required to determine frequencies. The numeric labeling is enhanced and based on OCR-A extended font, to maximize recognizability. Targets are offered on four standard substrates; There are four standard materials for each product; Chrome onGlass (CG) Chrome on Opal (OP) Photo Paper (RM) and Film (TM).