The B-tree works much the same for nonclustered indexes as it does for clustered indexes, but there are some differences. In nonclustered indexes:
Leaf pages are not the same as the data pages.
Leaf level stores one key-pointer pair for each row in the table.
Leaf-level pages store the index keys, data page number, and row number for the data row to which this index row is pointing. This combination of page number and row offset number is called the row ID.
The root and intermediate levels store index keys and page pointers to other index pages. They also store the row ID of the key’s data row.
With keys of the same size, nonclustered indexes require more space than clustered indexes.
