When can a transposition cipher be thought of as a substituion cipher?

[AXidenT]

Homework Statement

"Compare this cipher (transposition cipher) to a mixed (substitution) cipher and state under what circumstances a
transposition cipher can be thought of as a mixed cipher."

Homework Equations

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The Attempt at a Solution

For the first part of a question I stated that a mixed/substitution cipher changes the letters of the plaintext but not the order whilst a transposition cipher changes the order but not the letters of the plaintext (thus can be analysed through letter frequency count).

For the second part the only example I can think of is the obvious cipher where nothing changes, but I'm pretty sure that's not what they're looking for. Any clues?

EDIT: I tried taking a general example and got that they were only equivalent when the shift was zero and every letter was equal. I think I'm on the totally wrong track here. :/

Thanks!

 

[pbuk]

For the first part of a question I stated that a mixed/substitution cipher changes the letters of the plaintext but not the order

Are you familiar with a polygraphic cyphers? These substitute groups of letters so do not as such preserve letter order. How long could the group be?

 

[AXidenT]

So this condition would be a polygraphic cipher where the size of the group of letters that are substituted is the size of the plaintext? That would change both order and letter value, correct?

 

[pbuk]

That would change both order and letter value, correct?

Yes potentially, but for this to also be a transposition cypher then letter value would have to be preserved, which is rather contrived. I can't see what else they could be looking for though.