Or, to give their full titles, Associazione Calcio Milan and Football Club Internazionale Milano S.p.A.
Yes, that's AC and Inter to you and I.
Italy's top flight of football is fairly novel when it comes to the shared ground approach, which is a rarity in most other premiere competitions and utterly intolerable to many fans in the likes of England and Spain. Certainly the larger and more successful clubs and their fans rarely want to hear of the possibility of having to share a stadium with their biggest rivals.
But that's exactly what happens at the San Siro, where both teams play their home games.
So who exactly does the San Siro really belong to?
AC Milan were founded in 1899 and Inter Milan came along just nine short years later in 1908.
In fact, Inter came about because of a split in the ranks over the running of AC and, particularly, their signing of foreign players.
The San Siro stadium itself did not come along until 1926, when club president Piero Pirelli (yes, he of the car tyres) paid for the building of the ground for his team.
AC owned the ground from then until 1935 when they sold the stadium to the local council. For a further 10 years they were the sole inhabitants of the San Siro.
It was not until 1947 that Inter became joint-tenants in the stadium, and both clubs have played there since that day.
Verdict: AC Milan were there first, and would certainly lay claim to having had full ownership of the ground as another reason why the San Siro is "theirs."
Even before Inter Milan were formed, AC had wrapped up their first league championships, winning in 1901, '06 and '07.
A good headstart—but after the split and the subsequent formation of Internazionale, AC Milan did not win another league title until the 1950s.
Inter didn't take long to find their own success, with a top flight championship arriving in 1910, just two years after their inception.
Before AC's fourth title arrived in 1951, Inter had amassed five of their own already.
Between them, the two clubs won 11 Serie A titles in the 50s and 60s, with further success coming in the European Cup, which was won twice each—AC in '63, Inter in '64 and '65 and AC again in '69.
From 1970 until 2005, however, Inter won just three further league titles (and three Italian Cups), while AC Milan dominated the 1990s, winning five league championships that decade alone and adding three further European Cups in '89, '90 and '94. AC were also crowned the world's best club after winning the Intercontinental Cup in '89 and '90—though Inter's three UEFA Cups in the '90s is also to be respected.
Since the turn of the century it has been Inter, though, who have been the more trophy-laden team, with AC adding two leagues, one cup and two Champions League titles compared to Inter's five leagues, four cups and 2010 Champions League title.
Overall, it is almost neck and neck, with 47 total trophies facing off to 39 in favour of AC.
Verdict: Inter take this one. Eighteen league wins apiece is hard to call, but Inter's record five in a row was a magnificent achievement irrespective of circumstances. Inter lead 30-29 in terms of domestic trophies.
No doubt until this point plenty of you were wondering when the actual name of the stadium would appear.
Well, here it is: the "San Siro" is actually officially titled the Stadio Giuseppe Meazza, and has been since 1980.
San Siro is the district in which the stadium stands.
Meazza was an Inter Milan and Italy icon and legend, who won the World Cup twice with his country and played for 10 seasons in Serie A with Inter, scoring almost 200 league goals in the process.
He also played for AC Milan, for just two years, scoring nine league goals.
That he was one of Italy's finest all time players is undeniable, indeed one of the world's best at the time during the 1930s, but at club level he truly only belonged to one team.
Verdict: Giuseppe Meazza is Inter Milan, and so they take this area. For their fans, that is the name of the stadium. For AC, it will always be the San Siro—and, it must be noted, for a large number of non-Milano fans as well.
A tough one to call, given that numbers are based on sales of previous seasons and historical records, and that not every fan either purchases some kind of club-related merchandise or attends matches...but we'll do as well as we can.
Let's look at an indicative number: the average shirt sales per season.
Through 2005-2009, Inter Milan sold an average of between 400,000 and 600,000 replica shirts annually worldwide, which placed them at 9th in the overall club football listings.
AC Milan were placed 8th, just one ahead, and with an average yearly sales of the same number—400,000 to 600,000. It is expected that they turn out slightly more sales than Inter each year, though within the same bracket.
For the record, those two were the top-performing Italian sides.
Match-day gate numbers can also help tell us who attracts more visitors to the stadium.
There has never really been too much disparity between the numbers who see AC play, and who see Inter play their home games.
During the 80s and 90s, as AC Milan had their biggest and most impressive heyday at the top of European football they understandably saw far bigger crowds, while Inter saw similarly giant gates in the 1960s.
Since the turn of the century both teams have had a higher average attendance on six occasions apiece.
Verdict: Though the gap has not always been excessive, AC Milan impressively saw more people through the gates for their matches for 14 years in a row until 1998. That and the shirt sales points to more fans for the AC half of the city.
The San Siro is a well-known stadium with proud history and heritage, and the groundshare between AC Milan and Inter Milan is an often talked about point in conversations over footballing rivalries.
But that could all come to an end.
Inter Milan sold a minority stake in the club to a group of investors during 2012—and the end game appears to be to move out of their rented accommodation and into a purpose-built stadium of their own.
There’s a minority holding which is likely to increase as more investors join this group and the main aim is to build a new stadium – that’s what the agreement with China Railway is really about and it’s a very important one for Inter.
So spoke Massimo Moratti, the club president.
Verdict: Inter are only, and have ever been only, renting tenants of the stadium. Their wish to leave will see AC Milan in place as the sole club in the San Siro once more.
The San Siro belongs to one club, and one set of fans.
To the outsider, the name is irrevocably associated with the Rossoneri and, once Inter Milan complete their anticipated move away, they will once more be the only inhabitants of the grand old stadium, just as it was for years.
AC will, likely, have to vacate the stadium too at some point; it is old, lacking modernisation despite its renovations, and not offering the commercial opportunities that Italian sides need to compete at the top end of the European game—but any new stadium could be built on the same site, or keep the same name.
Who does the San Siro really belong to?