What's the difference between 平行 and 並行?


They both seem to mean "parallel". In my `プログレッシブ` dictionary, it seems to
point to the former being the more mathematical sense of the definition (two
co-planar, non-intersecting lines), while the latter indicates a more
metaphorical definition (doing things at the same time, parallel computing,
etc.).

However, my `大辞泉` has the following:

> 平行：「並行（へいこう）②」に同じ。（並行② is the metaphorical definition)

So which is it? Are they distinct like `プログレッシブ` says, or does `平行` encompass
`並行` like `大辞泉` says?




`平行` can mean the same thing as `並行` in the second sense (物事が同時に行われる), so it
does not encompass `並行` completely. For example, `電車と並行して走る` should not use
`平行` (although don't be surprised if you see the two mixed up). However, just
look at the characters, the 並 of 並行 can be seen in words like 並ぶ while the 平
of 平行 can be seen in words like 平面. So, in general, 平行 is primarily used in
the mathematical sense while 並行 is used to just mean "do two things at once"
or "line up and proceed together" like in `英語とフランス語を並行して勉強する` . Also, notice
that parallel in the computing sense is 並列, not 並行, which would be concurrent
computing.




`平行` can mean the same thing as `並行` in the second sense (物事が同時に行われる), so it
does not encompass `並行` completely. For example, `電車と並行して走る` should not use
`平行` (although don't be surprised if you see the two mixed up). However, just
look at the characters, the 並 of 並行 can be seen in words like 並ぶ while the 平
of 平行 can be seen in words like 平面. So, in general, 平行 is primarily used in
the mathematical sense while 並行 is used to just mean "do two things at once"
or "line up and proceed together" like in `英語とフランス語を並行して勉強する` . Also, notice
that parallel in the computing sense is 並列, not 並行, which would be concurrent
computing.

