Doubt on primary indexing and clustred index.

Question

I have read this. -

Clustered index are those in which data is arranged on fields so that it act as an index.  like the phonebook. And in non-clustered index we make a new index outside and data and indexes are at different places like book index at the back.

And this also . - primary index ( clustered ) secondary ( non-clustered)

so if the primary is clustered this means the index should not be present outside. But we many time implement the index outside in primary index does it not make it non-clustered ?

If primary index is a clustred index then what is clustred index that states that make a entry for unique keys only. Are they both the same?

Comments

Primary and clustered index are not one and the same..They have a similarity in the fact they are built on ordered attribute of a relation..

But the difference lies in the fact that the attribute which is chosen for building primary index must be a candidate key of the relation and hence repetition is not allowed..

On the other hand , clustering index is built on a non key attribute of a relation..

Also exactly one of them will be successfully implemented for a relation at a time but not both..

On the other hand , secondary index is referred to the index built on such attribute which is unordered as far as the relation(table) is concerned..

Unlike primary and clustering index , we can have multiple secondary indices on the same relation..

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I also was told the same but digging some more got me confused. Here is a question . what you think s1 is not right, 
https://gateoverflow.in/34682/indexing