A number of airlines experienced delays on Monday when their systems had a technical glitch, causing backups across the country. Travelers vented their frustration on Twitter, blaming airlines like Alaska, American and JetBlue.

But the real culprit was Sabre, a relatively unknown software system used by airlines for a variety of simple purposes, like tracking bookings and calculating baggage weight.

Sabre and other systems like it handle many crucial back-end airline functions, including taking reservations and scheduling flight crews. When its software works as intended, it makes it easier for airlines to operate smoothly.

“It's optimized for speed and reliability,” said Samuel Engel, a senior vice president and head of the aviation practice at the consulting firm ICF. “They often get referred to as the heart of the airline in the sense that they have arteries that touch every organ.”