If you’re looking into airlines, you’re looking into a different kind of stock.

You need to be able to figure out the most common causes of movement in airline stocks, what the acronyms stand for, and why some win and some lose. You want to look at the airlines the way Wall Street is watching them and know what details to watch closely.

First, it helps to understand the current state of the airlines industry. The early 2000s saw multiple huge mergers take place in an attempt to tame a rapidly growing and fairly wild airlines industry. At the time, there were fewer airlines servicing more airspace when compared to today. The airlines that survived the shakeup of the early 2000s were in an ideal position to raise ticket prices. This has led to the current situation, where Delta, Southwest, United, and American Airlines control approximately 80% of the US airlines industry. It was this process that brought Warren Buffett on board with the airlines industry, as he had previously called the industry a “death trap.”

There are three types of airlines in the US now, and these start with the big carriers like Delta, American, and United. Next we have low-cost carriers like Southwest and JetBlue. Lastly, there are ultra-low cost carriers like Spirit Airlines.

The airline stocks to look for are the ones that can report gains in unit revenue. Unit revenue measures the airline’s sales in relation to the airline’s overall flight capacity. A single unit of the airline’s capacity would be known as an ASM (available seat mile). There are other metrics as well, such as RASM (revenue per available seat mile), among several others. Now, you’d normally have to compare an airline’s income to the airline’s expenses, starting with labor and fuel, which make up the biggest portion of expenses. This is why unit revenue is a statistic that is more focused on by professional investors. Unit revenue can signal efficiency and profitability, or it can signal the opposite, so it is worth looking out for.