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