The ink has barely dried on the three-way merger agreement between CMS Cameron McKenna, Nabarro and Olswang but rumours are already circulating that an American firm may be joining the party.

It would be easy to dismiss any such move as wildly overambitious. Law firm mergers are notoriously hard things to put together – most talks break down long before making it to a partner vote. Actually integrating them is harder still.

And CMS Cameron McKenna is seeking to assimilate not one but two sizeable firms into its partnership, in the largest ever UK law firm merger. Its focus should therefore surely be on realising that deal, rather than trying to do another, right? Not necessarily.

That would be a fair assessment if CMS Cameron McKenna sought to directly merge with a US target. That is technically possible, but unlikely.

Like most large UK law firms, CMS Cameron McKenna operates an accrual-based accounting system with a 30 April financial year end, and compensates its partners via a modified lockstep. Most US firms, on the other hand, utilise a cash-based accounting setup with a calendar financial year and compensation systems based more on individual performance. Reconciling these differences in a conventional merger would be both complex and potentially costly.

That's why almost every recent major cross-border law firm combination has utilised the Swiss verein – a holding structure that allows member firms to join forces yet retain their existing forms. (The combinations that formed Dentons, DLA Piper, Hogan Lovells, King & Wood Mallesons, Norton Rose Fulbright and Squire Patton Boggs were all carried out via a verein.)

CMS Cameron McKenna could also pursue a verein-based deal. But that's not likely, either. In fact, the UK firm probably wouldn't be directly involved in any US combination at all. An American firm would instead be likely to join the CMS network – the pan-European legal and tax group with 60 offices and more than 3,200 lawyers, of which Cameron McKenna is just one of 10 member firms. If Cameron McKenna already has its plate full with Nabarro and Olswang, a US firm becoming the 11th CMS member would be another plate entirely.

You can't claim to be a truly global firm without a presence in the world's largest legal market

CMS was established in 1999 by Cameron McKenna and independent practices from Austria, Belgium, Germany and the Netherlands, and has grown over the years with the addition of firms based in France, Italy, Portugal, Spain and Switzerland.

The group has a number of centralised managerial and operational functions that distinguish the arrangement from that of a mere alliance network. CMS has a single overarching constitution and governance structure, with a global executive committee that determines firm-wide budgets and strategy. It also has a combined practice and sector group structure that runs across each of the 10 firms, with single leaders at a global – rather than regional or member – level.

That said, it is still a relatively loose affiliation compared to the majority of global law firm partnerships, with each member firm granted almost complete autonomy to run its own business and control aspects such as finances and staffing.

CMS is also structurally unique, as it is probably the only law firm to operate as a European Economic Interest Grouping (EEIG). EEIGs do not separate out members' liabilities, but are otherwise very similar to vereins. As the name suggests, US-based firms technically can't become members of an EEIG. However, a CMS spokeswoman confirmed that an American practice could enter into a contract with an EEIG to deliver "effectively the same outcome".

In May, CMS appointed former Cameron McKenna managing partner Duncan Weston to a new group role as executive partner for global development. Weston has shown an eye for opportunism – in 2013, he took a lead role on the firm's merger with Scotland's Dundas & Wilson. It would not be out of character for him to seek a US member to add to the group at some point. CMS has global aspirations – and you can't claim to be a truly global firm without a presence in the world's largest legal market, after all.