Why are し/ち transliterated as 'shi'/'chi', and not 'si'/'ti'?


In Japanese あいうえお are pronounced 'a i u e o', かきくけこ are pronounced 'ka ki ku
ke ko'. The spelling is simple and natural. However, when it comes to サ行,
さしすせそ, its relevant roman spelling is 'sa shi su se so'; the same issue also
exists in タ行, たちつてと, for which the roman spelling is 'ta chi tsu te to', which
makes me confused.

The question I want to ask is that why is it spelled 'sa shi su se so', but
not 'sa si su se so' for サ行, and 'ta chi tsu te to' instead of 'ta ti tu te
to' for タ行?

  1. Just to make its spelling correspond with the pronunciation? I don't think so, if you spell it phonetically like the book tells you, the pronunciation would be strange. 

  2. In order to let you type the relevant Japanese letters? If you type 'ta ti tu te to', for instance, you can also get 'たちつてと', without needing to type 'ta chi tsu te to'.




Systems of romanisation which were originally intended to render Japanese in a
way that makes it easier for foreigners to pronounce, like
[Hepburn](http://en.wikipedia.org/wiki/Hepburn_romanization), will use "shi"
and "chi" because those are closer to the correct pronunciation.

Other systems, like [Kunreisiki](http://en.wikipedia.org/wiki/Kunrei-
shiki_romanization), will use "si" and "ti" instead.

Which is used where is partly down to what the purpose is - Hepburn (or
variants of) is often used when the aim is to represent words for the benefit
of non-Japanese speakers. Kunreisiki is apparently popular in lingustics. [Wa-
puro romaji](http://en.wikipedia.org/wiki/W%C4%81puro_r%C5%8Dmaji) is for
those of us who are too lazy to change the way we type even when Japanese
input mode is off.

