Originally Posted by
findark
No, you cannot combine a Basic Economy fare with a non-Basic fare. It's in the fare rules.
Basic Economy does not need to be round-trip. However, yes, you must combine only BE fares.
To be clear, there are certain routes where the cheapest BE fares require a roundtrip purchase (and, as noted, there are also generally, but not always, BE fares with no roundtrip purchase requirements). On routes where cheapest BE fares require a roundtrip purchase, it may be cheaper to simply purchase roundtrip Main Cabin fares on one ticket instead separate tickets with BE one-way and Main one-way if you just want BE in one direction. For example, DTW-MSY is available for $278 in Main Cabin roundtrip when booking roundtrip fares. While booking cheapest one-way BE and one-way Main fares would be $159 + $189 = $348.
An example of a route where there are roundtrip BE fares, but no one-way BE fares is DTW-CHS. The cheapest one-way fares on DTW-CHS are $219 and are in Main Cabin (T basis). While there are BE fares on this route from $318 roundtrip (Main Cabin roundtrip fares start at $373 roundtrip).