The correct idiom is Not only... but also... .

Hence But also should always follow Not only.

but also can follow without not only . To understand better,Refer to following generics:

" X but also Y" =>> X & Y are contradicting views
"Not only X but also Y" =>> X & Y are reinforcing views

ccassel wrote:MUST the idiom "not only" always be followed by "but also"

not necessary.

My revision from Ron's post,

Not .. But .. -> basic idiom

Only, also are modifiers and to be used based on meaning.

Thus, the sentence can be,

Not X but Y - Contrastion views.
Not only X but Y - Reinforce each other.
Not only X but also Y - Reinforce each other and one goes beyond the other.

The above three sentences differ in meaning.

is it still gramaticaly correct to have "not only" without "but also" preceeding it?

I dont think so. I have actually never come across such construction.


Lets see what experts have got to say.

ccassel wrote:MUST the idiom "not only" always be followed by "but also" or is it still gramaticaly correct to have "not only" without "but also" preceeding it?

An experts reply would be very much appreciated!

Thanks,

If NOT ONLY is present then BUT ALSO MUST be present.
Whereas BUT ALSO can be present without NOT ONLY also, but vice versa is not true.

Correct forms are:
X BUT ALSO Y
NOT ONLY X BUT ALSO Y

I hope this helps

Thanks