On any taxiway that connects to a runway and carries a Pattern A runway holding position marking. At Part 139 airports that means every taxiway that intersects a runway (14 CFR 139.311(b)(1)(ii), ¶4.1).

It is never painted on a runway, including a runway temporarily in use as a taxiway, and never on taxiway shoulders. It also does not belong on a taxiway that does not lead directly onto the runway, such as one crossing an approach area. (¶4.5.1, ¶4.5.2.1, ¶4.5.2.2)

On taxiways over 35 ft, the circular names two special situations that justify going below 9 ft: a taxiway narrower than 75 ft (22.9 m), or a taxiway that has to display multiple runway designations with arrows. At 35 ft or less, only the multiple-designation case applies. Reductions are per entrance, not per runway, so every other entrance to the same runway that does not need the reduction stays at the standard height.

If the inscription still will not fit at the floor, break the marking into stacked signs, one runway per sign, one above the other, with 2 to 4 ft (0.6 to 1.2 m) between the stacked inscriptions, then raise the height back toward the standard. Figure D-15 gives the recommended order of appearance. Stacking doubles the sign count, so it doubles the character order. (¶4.5.2.4; Table D-1 notes to Fig. D-5; Table D-2 notes to Fig. D-6; Painting Note No. 9; Fig. D-15)

On taxiways over 35 ft, the base case is one sign to the left of the centerline. A second on the right is strongly recommended wherever operations rely on two-person crews, and it may be omitted only when the centerline is closer than 45 ft to the taxiway edge and a vertical mandatory holding position sign is in clear view. At 35 ft or less, one sign, centered over the centerline. Where centerlines converge within 15 ft of the hold line, a wide taxiway may omit one side and a narrow one takes a single sign centered over both.

Two conditions add signs rather than reduce them. An entrance wider than 200 ft (61 m) gets a repetitive pattern of additional signs, spaced by the Airplane Design Group of the Runway Design Code rather than the Taxiway Design Group: ADG I at 69 ft, ADG II at 105 ft, ADG III at 152 ft, ADG IV at 215 ft, ADG V at 267 ft, ADG VI at 324 ft, each measured from the same outer edge. And on ADG III through VI runways with insufficient lead-on length, two signs are mandatory, one on each side of the centerline.

Lead-on length is insufficient when the design airplane cannot line its whole fuselage up perpendicular to the holding position marking, or on a straight section of centerline at it. Three tells are visible on the pavement: centerlines that only curve into the marking, markings painted in or near fillets, and an enhanced centerline noticeably shorter than 150 ft. (¶4.5.2.4.1, ¶4.5.2.4.2, ¶4.5.2.5, Safety Notice No. 9, Table 4-1)

The inscription is the runway designators, identical to the vertical runway holding position sign, with the same numbers, letters and arrows. If the entrance serves two or more runways, the sign has to show every runway designator plus directional arrows approximating the orientation of those runways. That requirement is usually what drives a height reduction or a stacked layout, so resolve it before choosing a size. (¶4.5.4.2, Safety Notice No. 8)

The red panel is rectangular and extends beyond the extremities of the inscription, horizontally and vertically, by 15 in (38 cm) on taxiways over 35 ft and 7.5 in (19 cm) at 35 ft or less. On taxiways over 35 ft that margin is never reduced, even when the inscription height is. (¶4.5.4.3, Tables D-1, D-2)

Placement is prior to and parallel to the Pattern A marking, 2 to 4 ft (0.67 to 1.34 m) away on taxiways over 35 ft and 2 to 3 ft (0.67 to 0.91 m) at 35 ft or less. Laterally the sign sits 3 to 10 ft (0.91 to 3.0 m) from the centerline on wide taxiways and centered on narrow ones, always at least 2 ft from the inside taxiway edge marking or the pavement edge. Leave clearance for in-pavement runway guard lights and stop bars. (¶4.5.2.3, ¶4.5.4.4, Table D-3)

The black outline goes around the sign on light-colored pavement, but the black outline that surrounds the white inscription on the vertical holding position sign is not required on the surface painted version. It is a real difference between the two signs, and one fewer stencil operation in the field. (¶4.5.3)

Glass beads in the red background are Type I. Type III beads wash out the background color and reduce the contrast between the inscription and the panel. Beads are never applied to black or green paint. Application rates follow Item P-620 of AC 150/5370-10. (¶1.4)

Every character has to be the FAA Appendix B letterform from AC 150/5340-1M. Heliport letterforms in AC 150/5390-2D are geometrically different and cannot substitute for an airport inscription.

One thing worth planning for: if the taxiway width later changes across the 35 ft boundary in either direction, the existing sign has to be physically removed and repainted in the other configuration, centered becoming left-of-centerline or the reverse, and the enhanced centerline dashes usually go with it. A width change is a re-stencil, not a touch-up. (Painting Note No. 8)