No. 2 Translation NotesBeware of ファイ/φ/Φ and アール/R, False Friends from DrawingsFriends on ISO-conforming drawings, but not your friends in running text

William Lise (October 24, 2023)


It appears that a technology problem in rendering the ISO drawing symbol for diameter, ∅ has resulted in people thinking that the character is the Greek letter phi and this symbol is actually rendered variously and uniformly incorrectly in running Japanese text as φ (lower-case Greek letter phi), Φ (upper-case Greek letter phi, or ファイ. All of these renderings are wrong. The correct character is ∅, which is evoked in HTML by ∅ and in Unicode by U+2300.

Although both ファイ and some of the above-noted symbols (mostly the incorrect ones) are used in running Japan text in place of the word diameter, this is simply wrong in English, even if you use the correct symbol, ∅. Use the word diameter in running text and the abbreviation DIA in drawings in cases in which the ISO symbol ∅ would not be understood (probably mostly in the US).

None of the bogus symbols should appear in running English text.


  • φ5の棒 {a 5-mm-diameter rod}

In an ISO-conforming drawing of that rod, of course, the dimension would be indicated as ∅5.


Here we have not a special character problem, but simply a misunderstanding the R (and its katakana rendering アール) can and should be used to mean radius. R is the ISO drawing symbol for radius and has little proper use elsewhere. In a drawng, R10 means a radius of 10 mm. Some Japanese authors will even use R as a faux unit. When asked, they might explain it as ミリ直径. This is wrong and results by combining R with a drawing convention that, unless otherwise noted, dimensions are in millimeters.

R should not be usable in English to simply replace the word radius.


  • コーナーにアールをつける。{Round off the corner(s).}
  • Rは小さすぎるので、... {Because the radius is too small, ...}