These firms can get overlooked by students pursuing a legal career, but they often offer graduate opportunities that mix some of the best aspects of working at either extreme of the legal market.
Here we take a look at these firms and why you might want to work at one, with the help of a lawyer who already does.
In some ways, the work of a medium-sized commercial firm is similar to that of a large commercial or City firm.
Most lawyers here will be acting for businesses, helping them with mergers and acquisition deals, to find financing, to purchase and protect assets, and to resolve disputes. However, there are some significant differences.
The client list is likely to be more diverse at a smaller commercial firm. It'll usually include a good mix of small businesses and startups, public sector organisations, charities, and media and technology firms, alongside some work for the big international corporates and financial institutions that larger firms tend to focus on.
Many firms in this sector also take on private client work, which means advising high net worth individuals, often with international financial interests, on the legalities of areas including wealth management, migration and domicile issues, and the sale and purchase of valuable assets.
Private client work is a type of work not often found at larger commercial firms, and the lawyer at a medium-sized commercial firm that we spoke to says that she thinks the fact that her firm takes on a lot of private client work is incredibly beneficial for the firm as a whole.
"A lot of private client work is about getting to the heart of the specific and unique needs of your client. And I think that's a lesson that has really spilled over from the private client department into the rest of the firm."
"We're always trying to gain particular insights about our clients to help us add value when designing legal solutions, because no client is standard."
You're also likely to find a greater emphasis on real estate work at a medium-sized commercial firm, with lawyers working in this area having their own clients and independent deals, rather than spending much of their time advising on property aspects of corporate and banking deals as their equivalents at large City firms do.
"Niche" areas of practice such as employment law, media law or technology law may also be high-profile at a medium-sized firm. A good number of firms specialise in one or more of these areas, and some are regarded as the best in the UK legal market in their area.
"You often work directly with a partner as an NQ or trainee," says the lawyer we spoke to. "You get really good client contact from the beginning and a very nice mix of responsibility and supervision. I think that's something that sets a mid-sized firm apart."
Being part of a smaller team on a project can also be good for your career progression prospects, as it gives you a great opportunity to start building the client relationships that become so important as you develop into a senior lawyer.
Another benefit for your career of working at a medium-sized firm is that you have a better chance of getting to know your colleagues across the firm personally, which is helpful when you're trying to decide what seat to do next, or need advice from another department for something you're working on.
"If you're one of 80 trainees," says the lawyer we spoke to, "you go through the two years and never even meet all the others in your cohort. But trainees here get to know all of their cohort, and get exposure to a lot of other people firm-wide as well."
And if there are people that you don't manage to meet naturally in the course of your work, approaching them is easy at a medium-sized commercial firm, our lawyer has found.
"The type of firm we are gives us the ability to go up to someone we've never met and strike up a conversation, whether they're more senior or more junior."
Anyone considering working at a commercial law firm needs to consider the kind of lifestyle they want.
Wherever you end up, you're unlikely to always work a regular 9am-5pm Monday to Friday week. But demands on your time will vary depending on your firm, and the hours at a medium-sized commercial firm tend to be a bit more palatable than those demanded at the largest City employers.
"Generally speaking," says our lawyer, "I have my evenings and weekends free, but it does depend on what I'm doing. If you're on something big, you're expected to work hard, and rise to the challenge."
"I was on a really big case last year and was working weekends and even on holiday. But if you're working on holiday, sometimes you can get time back in lieu, and I think in general here there's a very respectful approach to employees' leisure time or holiday time."
Finally, adds our lawyer, another thing she likes about working at her medium-sized commercial firm is its tolerance for individuality.
"I'm sure there are opportunities to stand out at a larger firm, and, like them, we work as an integrated team. But I do think that individuality and individual personalities are sought out, championed and supported better at a mid-sized firm."