Have you ever noticed the letters, or numbers, painted above the cockpit windows on the front of some aircraft? These are not present on all aircraft but are different between each one. They are nothing too mysterious though. They are simply part of the aircraft registration, placed to aid aircraft identification on the ground.
Shortened form of the registration
Look at the front of the aircraft and above the cockpit; you will often (but not always) see two or three letters or numbers. These may at first seem confusing and arbitrary, but they are not. They simply represent the unique characters of the aircraft registration number. There is no standard for what part of the registration is used - or indeed, that anything appears at all. Airlines will normally choose the part of the registration that uniquely identifies the aircraft.
The aircraft registration is usually displayed on the side towards the rear of the fuselage. This is fine if you are viewing the aircraft from the side, but what about the front? Repeating part of the registration above the cockpit aids in identification on the ground. If you have several aircraft on the ground, at gates, or lined up for maintenance, it is helpful to identify the aircraft from the front.
Shortening registrations like this is common in radio communication too. General aviation aircraft will often identify themselves, after initial contact, with just a shortened form of the registration ending characters.
Aircraft registrations
Aircraft registrations can be confusing - not least because they can change several times during an aircraft's operational life. This typically happens whenever the aircraft changes owner, and certainly if it changes country. Registrations are formally known by the International Civil Aviation Organization (ICAO) as Aircraft Nationality and Registration Marks.
The registration is made up of two parts. The first one or two-character prefix identifies the country where the aircraft is registered. The suffix of one to five characters uniquely identifies the aircraft. Sometimes these are assigned in order for a series of aircraft, but there is no fixed rule. The characters we see above the cockpit will be the part of the suffix characters that identifies the aircraft within the type.
Take British Airways' 787-8 aircraft as an example. The 12 current aircraft carry sequential registrations from G-ZBJA up to G-ZBJM. Any UK-registered aircraft will share the 'G' prefix. The British Airways A380s, as another example, take sequential registrations from G-XLEA up to G-XLEL.
Not all airlines follow this sequential pattern. Some are more scattered - and of course, it is not as easy when fleets keep changing. And some airlines have taken a more light-hearted approach. Icelandic airline WOW Air, for example, chose a lovely family series for some of its A320s and A321s - with registrations TF-SIS. TF-BRO. TF-MOM, TF-DAD, TF-KID, and TF-SON (TF is, of course, the prefix code for Iceland).
So now you know what those letters are when you look at the front of an aircraft. Next time you see them, check the tail registration to see what pattern the airline is using. Feel free to discuss further or share any unusual registrations that you have seen in the comments.