ALTER statement: Why VARCHAR2(50 BYTE) instead of VARCHAR2(50 CHAR)?

Question

I executed the following (Oracle 11g) SQL statement to increase an existing column's length from VARCHAR2(20 CHAR) to VARCHAR2(50 CHAR):

ALTER TABLE USERX.MY_TABLE MODIFY (LASTNAME VARCHAR2(50));

It succeeded without incident, but when I look at the new Data Type column, I see: VARCHAR2(50 BYTE) instead of VARCHAR2(50 CHAR).

My questions are:

1. Why BYTE and not CHAR? What have I done incorrectly?
2. How do I fix the column's length to be VARCHAR2(100 CHAR)?

Solution

Answering myself (thanks to the tip provided by this other answer):

I should have executed instead:

ALTER TABLE USERX.MY_TABLE MODIFY (LASTNAME VARCHAR2(50 CHAR));

(note the extra CHAR after 50)