SQL Server UNION - What is the default ORDER BY Behaviour

If I have a few UNION Statements as a contrived example:

SELECT * FROM xxx WHERE z = 1
UNION 
SELECT * FROM xxx WHERE z = 2
UNION
SELECT * FROM xxx WHERE z = 3

What is the default order by behaviour?

The test data I'm seeing essentially does not return the data in the order that is specified above. I.e. the data is ordered, but I wanted to know what are the rules of precedence on this.

Another thing is that in this case xxx is a View. The view joins 3 different tables together to return the results I want.

There is no default order.

Without an Order By clause the order returned is undefined. That means SQL Server can bring them back in any order it likes.

EDIT: Based on what I have seen, without an Order By, the order that the results come back in depends on the query plan. So if there is an index that it is using, the result may come back in that order but again there is no guarantee.

In regards to adding an ORDER BY clause:

This is probably elementary to most here but I thought I add this. Sometimes you don't want the results mixed, so you want the first query's results then the second and so on. To do that I just add a dummy first column and order by that. Because of possible issues with forgetting to alias a column in unions, I usually use ordinals in the order by clause, not column names.

For example:

SELECT 1, * FROM xxx WHERE z = 'abc'
UNION ALL
SELECT 2, * FROM xxx WHERE z = 'def'
UNION ALL
SELECT 3, * FROM xxx WHERE z = 'ghi'
ORDER BY 1

The dummy ordinal column is also useful for times when I'm going to run two queries and I know only one is going to return any results. Then I can just check the ordinal of the returned results. This saves me from having to do multiple database calls and most empty resultset checking.