That being said, there is a method to the madness, and their award calendar and agents typically price Skymiles awards according to the same rules.
Understanding Delta's Award Fare Buckets
Delta prices each class of award in five different buckets. For example, an economy segment can be booked in N, NL, ND, NS, or NK. N is the cheapest level, formerly known as Saver. NK is the most expensive level, formerly known as Peak.
Conveniently, Delta's search engine displays the fare bucket in their search results.
Finding the cheapest flight or segment is easy! As long as you're book a flight in a single-letter bucket, you know you're getting the best price for a flight or segment.
With most airlines, you can go online, search for each segment of your flight, then simply string together the lowest priced segments to get a cheap flight. Delta won't let you do that. As a matter of fact, if you try to string segments together on Delta's website, the pricing will always be additive.
Instead, Delta flights are often priced using Married Segments. What this means is GRR-DTW-JFK may price out as a Standard award even if you can book GRR-DTW as a Saver award and DTW-JFK as a Saver award.
Here's a real-world example. Say I want to fly from MBS to ATL via DTW. First, I look up the segments individually and find both at the N (Saver) level:
If the flight is operated as a Married Segment, you must book it as a Married Segment, or you'll need to add the two segments together. In this case, Delta might let you book it as two segments, but you'd have to pay 25,000 miles (12k+12k). I'll take the Married Segment for 20,000, thank you.
Partners Don't Use Married Segments
Fortunately, most (all?) of Delta's partners do not use Married Segments, so if you're booking on a partner you can piece the flights together exactly how you'd expect with other award bookings. Just keep in mind that any connecting Delta flights will price out using Married Segments.
The best way to avoid Married Segment pricing is to avoid connecting Delta flights altogether.
Voluntary Downgrades and Safely Breaking Married Segments
Earlier, we talked about the fare buckets Delta uses for award travel. Here's the same chart with domestic award pricing as of 2015:
Delta will typically allow voluntary downgrades on award tickets. A voluntary downgrade involves taking one segment of a higher class trip in a lower class of service. This is great for a couple of reasons. First, Delta's domestic availability is terrible, so downgrading a domestic segment is often your only option for finding a Saver Award. Furthermore, downgrading a segment is often a great way to break a Married Segment price and find Saver level availability.
The thing to remember with downgrading is you must stay at the same level to avoid additive pricing. In other words, you can combine R, O, and N segments and pay 25,000 miles. Or you could combine RD, OD, and ND segments and pay 37,500 miles. Once you break this rule, Delta's engine starts adding things together.
Downgrading a segment will break Married Segments, pricing at the highest segment cost instead of adding the segments together.
Here's a real-world example from Delta's site. Can you guess the price in first class?
This one in RD, OD, and RD will price at 37,500 miles.
Just to reiterate, if you don't stay at the same level, things get bad. The example below of a similar itinerary has a segment in RD and two in RK. However, because I'm no longer staying at the same level, it combines the price for the RK segments and adds on the RD segment for a whopping 92,500 miles (55k + 37.5k)!
How to Avoid Additive Pricing
Again, all you need to do is stay at the same level to avoid additive pricing.
All partner flights book at the Saver level (R, O, N), so be sure to book your domestic flights or Married Segments in R, O, and N class as well. This one in R, O, and O books properly at 70,000 for an international flight to Seoul on Korean Air and Delta:
As soon as you combine levels, things get nasty. Here's a similar flight in RL, RL, and O. The first two segments get combined and the final segment gets added. Therefore, this flight adds up to a massive 102,500 (32.5k + 70k) miles:
Can Additive Pricing be Good?
The only good news here, if there's any, is that additive segments are each priced based on their segment regions. So, if you combine a Saver Korean flight in O with Peak Delta domestic flights in NK, you'd pay 102,500 (70k + 32.5k) instead of having everything bumped up to the Peak level which would cost an insane 325,000 miles! Yes, there are plenty of times when it makes sense to utilize additive pricing.
You can always force additive pricing by using Delta's online search engine and putting in the segments you want, however I've found that their search engine usually prices this way by itself without any prodding.
In summary, when booking Delta awards, keep an eye on the level. You must stay at the same level to avoid additive pricing. If you combine segments from the same level and you don't break any Married Segments, you'll simply pay the price of the top priced segment. However, if you combine segments from different levels or break Married Segments, you'll pay the additive price of all segments.
In some cases, voluntarily downgrading a segment is a good way to stay at the same level or break a Married Segment without making the price additive.
In some cases, it makes sense to invoke additive pricing.