Let’s say you’ve recently bought a brand new chronograph. You look down at your wrist with with pride, ready to conquer the world, when you notice these strange lines in the minute counter at the 3-, 6- and 9-minute marks. You thought you did your research and knew everything there is to know about your precious timepiece, but you are stumped. What do these strange lines mean; who came up with them; and why are they on your watch?

Granted, not all chronographs have this odd distinction, so don’t feel bad if you’ve never noticed them before. Most modern chronographs have conventional 30-minute counters, highlighting each five-minute mark with a bolder or elongated hashmark. However, in our world of endless homage releases and vintage-inspired timepieces, one will find numerous examples of these in the wild, such as the Blancpain Air Command and the Longines Avigation Big Eye, among others.

Just imagine; it’s the mid seventies, you have one phone in the house, and it’s a rotary phone that you have to share with everyone — how did we ever survive? — and making a phone call is expensive, very expensive, especially long distance and in particular calling overseas. Remember, we are not in the WhatsApp age, yet. And here is the kicker: your summer crush, that Swedish au pair you love so much, had to go back home and the only way to stay in touch with her is to call her under the watchful eyes of your father with his trusty chronograph.

One can’t really imagine that the esteemed engineers at aviation-inspired manufacturers, such as Blancpain, Breitling, Longines and others would have condoned such a nuisance application for an otherwise meticulously crafted Swiss timepiece. It is further interesting to note that in the mid 50s, Breguet would produce watches exclusively for military Air Force units specifically using three-minute increments on a 30-minute or 15-minute totalizer. For one possible explanation, think Indy. Indiana Jones.

Three-minute as well as six-minute increments can be used to quickly calculate rates, such as distance and speed, as three minutes is exactly 1/20 of an hour and six is 1/10 of an hour. Therefore, they are much easier to work with than five-minute or 10-minute increments, which would equate to 1/12 of an hour or 1/6 of an hour respectively.

Since Indy’s speedometer is blown to bits, he needs to figure out his pace by quickly calculating the distance in yards his plane travels in three minutes, which is about equal to the speed in knots divided by 100. Let’s assume he travels one mile per minute, equaling three miles over three minutes, totaling 5,280 yards, which in turn equates to 52.8 knots. Oh dear, that won’t do it, he needs to get to 60 knots to take off. But lo and behold, Indy comes through, and he escapes into the sunset.

These types of “on the fly” calculations could be lifesaving for pilots at a time when the use of electronics was scarce, and time was of the essence. So, it would make sense to imagine that the three-minute increments had purposefully been applied to the minute chronograph counter for that exact reason.

The last theory on this subject is that the 15-minute versus the more conventional 30-minute sub-dial was intended for aviators to time their final plane check before taking off — but that just sounds a bit too simple.