The Signal Square was neither fish nor fowl. It was installed and maintained at aerodromes by Airports staff, and operated by the resident groundsman or airport fireman on the instruction of the air traffic controller or flight service officer.

The Signal Square, or signal area, contained symbols to indicate visually to over-flying aircraft conditions on the aerodrome. It had pre-War origins, supplementing an earlier set of ground signals, and was intended for aircraft that carried no radio communications equipment (which included most light aircraft well into the 1960s). As soon as ICAO was created in 1947, Annexe 14 contained (and still does) instructions on the size and construction of the Signal Square. Annex 2 describes its use by aircraft. Ground signals are still used at aerodromes to this day – the landing T and the gliding-in-operation symbol being the most common ones, but the general use of the signal square had fallen into disuse by the late 1950s when VHF air-ground communications had been largely introduced.

The square was marked out on the ground in the vicinity of the control tower and the wind- sock. It had sides 40 feet long, with a white-painted border. The surface inside the square was smooth and level, surfaced in bitumen, ashes, sealed gravel, turf or even bare earth. It was usually blackened with sump oil or black ashes to provide a contrasting background to the symbols that conveyed the information. The were four basic signals; the red square signal, the dumb bell signals, the landing T, and the right hand circuit indicator. All of these objects were made of timber and painted black on the reverse side. They were made of a series of hinged sections so that they could be folded up when not in use, and would not be visible from the air as the reverse side was showing, and appeared black.

The function of these elements was as follows.

Red Signal Square:
A red square with no diagonals indicated that the aerodrome rules were temporarily suspended, and that special arrangements were in operation. If this square had one yellow diagonal cross-bar, it indicated that the bad state of the landing ground or a temporary obstruction required caution by the pilot when landing. If the Red Square had two yellow diagonal cross-bars, this meant a total prohibition of landing for a prolonged period.

Dumb Bell Signals:
These were only displayed only on aerodromes with sealed runways. A white dumb bell indicated that the airfield surface was unserviceable except for the runways and that take-offs and landings were to be made on the runways only, and that all taxying must be on paved surfaces only. A white dumb bell with black bars superimposed across the centres of each circular portion of the dumb bell meant that landings and take-offs must be on runways only, but grass surfaces may be used for taxying, subject to standard serviceability markings. A red "L" on the dumb bell indicated that light aircraft may land or taxi off the runways, but that heavy aircraft must use runways only.

Landing ‘T’:
This was probably the best known of the signals, and was used at all-over landing fields. It was mounted on a pivot for ease of turning, and supported on castor wheels or skids. It was lined up with the wind direction, and an aircraft landed along the shaft of the "T" towards the cross arm. While not commonly used, a white disc at the centre of the cross piece of the "T" indicated that take-offs and landings were being made in more than one direction, and that pilots must be ‘on the alert’. Of course, the air traffic controller or Aeradio operator on-duty had to vigilant, and change the direction of the T whenever the wind direction changed.