The question I get most about the Thước Lỗ Ban app: “Why three rulers? Which one do I use?” Short answer: each measures a different class of object, and using the wrong one invalidates everything downstream.
The Thông Thuỷ ruler
For clear openings — the aperture of a door, window or vent. The key point: measure what light and air actually pass through, not the outer frame. This is the most commonly mismeasured dimension.
The Dương Trạch ruler
For dwellings of the living and household objects: table heights, cabinet depths, counter and bed dimensions. This is the ruler used most often in construction and interior work.
The Âm Trạch ruler
For gravesites and spiritual structures: tombs, caskets, altars. The same number lands in a different segment than it would on the Dương Trạch scale — which is exactly why they are not interchangeable.
The eight segments
Each ruler repeats eight segments: 財 Tài, 病 Bệnh, 離 Ly, 義 Nghĩa, 官 Quan, 劫 Kiếp, 害 Hại, 本 Bản. The four auspicious ones are Tài, Nghĩa, Quan and Bản.
The part rarely said out loud: landing in a good segment is necessary, not sufficient. A door with perfect dimensions but the wrong orientation or the wrong place in the overall layout is not saved by a pretty number. The ruler is the final check, not the starting point.
