"There were obviously synergies, but most importantly there was a feeling that we both wanted to practise as one firm and not be a verein. That is where we began – the other pieces just fit together," says Therese Pritchard, co-chair of the newest transatlantic law firm, Bryan Cave Leighton Paisner, of the firm's single profit pool structure.

Her UK opposite number, Lisa Mayhew, is equally adamant that financial integration was a prerequisite. Highlighting the 2007 Reed Smith-Richards Butler tie-up as the most recent example of an integrated transatlantic merger of a comparable scale, she insists a deal could not have happened without it, despite the additional time and cost involved.

"From the first conversation we were both only interested in a properly integrated, combined new firm, not a verein," she says. "We genuinely believe that the service you get as a client is more seamless and efficient if you institutionalise the 'one team, one firm, one culture, one profit pool' mantra."

While rivals unconnected to either firm speculate that the one-off tax bill associated with the move from the UK accrual accounting system to the cash-based system of the US is likely to run into tens of millions, the pair will not be drawn on how much the decision will cost them.

"We view it as a deal cost," is all Pritchard will say of the bill, which is set to be shared across partners from both the UK and US arm over a number of years.

"By spreading the cost, this is effectively just a paper exercise," adds Mayhew. "The value that we are convinced is created through having one firm – in terms of the greater incentive for partners to share clients and chase new relationships – should more than compensate, so that revenue growth will be greater than the cost that flows forward."

However, neither denies the additional complication it caused, with tax wrangles delaying the vote by several months.

Though the timeline from talks to full financial integration of a 1,600 lawyer, $900m firm looks to have been relatively smooth from the outside, both women clearly feel otherwise, pointing out that it has been a full 14 months from their first meeting to launch date.

The bulk of partners learned of the discussions about eight months after the initial meeting between Pritchard and Mayhew took place, just days before news of the discussions broke in the press in October.

Though partners and staff from both the legacy UK and US sides are all now getting to grips with a very different new employer, it is UK partners who have most change to contend with – switching from 1 January to a new remuneration structure that closely resembles that of legacy Bryan Cave.

The value that we are convinced is created through having one firm should more than compensate

Though management are at pains to stress that individual partner pay is unlikely to look much different, the forward-looking, non-lockstep-based structure represents new territory for UK partners.

Says Mayhew: "We were very inclusive in the way we approached the discussions, asking partners from each of our firms to form a working group. This was an example of creating a different system, which we feel will be fair in terms of being able to compensate our highest contributors, but that is also transparent."

She insists that partners are undaunted by the shift, which will see pay decided by a remuneration committee that, in the short term at least, is likely to reward integration efforts.

Pritchard adds: "When you look at what is underlying [the two remuneration structures], it was very similar – driving revenues, profitability of work, participating in teams in a way that means clients return, doing great work, billing hours, being a good 'firm citizen'. Even without lockstep, you could give confidence that remuneration wouldn't be cut by 80% if they didn't perform one year – there's a human element to it, and we understand that."

The fact that both firms have a very different geographic reach, with Bryan Cave predominantly based in the US and BLP boasting a more global presence, means integration efforts are being focused very much on an international practice and sector level, with a team of partners tasked with fusing their respective operations.

Secondments will be pushed heavily, with the firm also set to adapt the 'buddy' system used by Bryan Cave in several of its earlier US mergers.

The combined firm will be organised under four global departments – corporate, finance, litigation and corporate risk, and real estate. The UK arm of the firm is seeking to bolster its corporate strength, while Pritchard acknowledges that real estate is likely to be a key area for growth in the US.

On a geographic level, priorities for expansion will include New York and Washington DC, as well as continental European destinations such as Paris.

Beyond this, technology and innovation will be high on the agenda, given each firm's history in this area, as well as diversity, which will unsurprisingly be a core focus for one of the few major law firms with female leadership.

In the short term, however, plans appear fairly simple.

As Pritchard concludes: "Integrate, integrate, integrate. It's about doing everything we can to help people get to know each other, each others' clients and institutionalising the collaboration that will drive the future of the firm."