As per IS standard specifications for timber door, window and ventilator frames, the symbolic designation of 12 DT 20 would indicate:
1
frame of window shutter door with a width of 12 modules and height of 20 modules
2
frame of ventilator with a width of 12 modules and height of 20 modules
3
frame of defined shutter door with a width of 12 modules and height of 20 modules
4
frame of double shutter door with a width of 12 modules and height of 20 modules