What do commas in Cypher do?

General Newbie Questions
1

For example:
MATCH
(a)-->(b)<--(c),
(c)-->(d)

Or

MATCH ()-->() MATCH ()-->() RETURN 1
versus
MATCH ()-->(), ()-->() RETURN 1

2

Commas separate parts of the same pattern.

Separate MATCH statements are different patterns.

The main implication of that is relationship uniqueness is guaranteed within the same pattern but not across different patterns.