See: Roll Up.
A roll-up is really more of a series of acquisitions than a merger. Acquisition = Buying Stuff (usually with cash, but sometimes with stock).
So now think about buying things, or rolling them up, where the targets are about the same size as the acquiror. That's where the notion of a roll-up merger comes in. You're kind of buying, kind of merging a series of companies together for some logical reason.
Like...if you could buy all the hotels in a given 40-mile strip of highway, and then raise prices on all of them 20% (and not get regulated), you could take profit margins and revenues up so much that it'll have been worth the various frictions you undertook in buying the various targets you hit. You'll have diluted yourself in issuing stock for some of the purchases, but at least notionally, the "one plus one" event of this roll-up merger will have been way more than two.
See: Synergy (albeit skeptically).
Related or Semi-related Video
Finance: What is The Difference Between ...6 Views
Finance allah shmoop What is the difference between a horizontal
merger and a vertical merger Okay Mergers let's talk rock
As in a feller he was kind of the king
of mergers both vertical and horizontal Let's Talk about what
comprises each of these things All right in the energy
industry specifically oil Ah horizontal monopoly would exist if a
company owned all the oil wells in the world And
in fact for a short time opec owned well it
was very close to a monopoly at least an enormous
percentage of all the oil wells in the world such
that they were able to constrain supply create panic and
increase prices dramatically some five hundred percent and change the
world during the nineteen seventies when we had a very
weak president going against them and here's what inflation adjusted
prices for a barrel of oil looked like in that
period So that's a horizontal monopoly like where you own
all the sources of oil coming out of the ground
horizontal So what's a vertical monopoly Well in the process
of processing oil a lot has to happen for the
system to work right first step you have to pull
All the oil out of the ground right the oil
well but then you have to process it or synthesize
it from dinosaur coop into well something that's actually usable
in your lexus with the turbo engine Then because the
world demand is continuous you have to store the oil
and then distributed continuously forever and ever and ever and
eventually the retail customer buyer has to be ableto pull
up into a gas station think real estate here and
fill her up So if you owned a vertical monopoly
while you would own the discovery and mining of oil
the synthesis or processing of it or refining of it
as it's called in the industry you don't a storage
company a trucking and distribution company and while then a
bunch of gas stations well that would be a fully
integrated vertical monopoly So when horizontal and vertical mergers get
discussed they get framed under this format So let's say
we're coric coffee machines and we want a vertical merger
in our business because we're sick and tired of paying
coffee growers twelve cents a cup for something well that
cost them less than a penny So we at keurig
Decide to buy our own coffee plantation roasting and grinding
and processing company so that we can supply our own
coffee in our own little cups Well that would be
a vertical merger in the coffee business And it often
makes a lot of sense because all that profit that's
been given out to coffee vendors selling to the kindly
loving caffeinated folks at koi rig with then be capped
and retained by the kindly loving shareholders of keurig vertical
versus horizontal Good ways to emerge and good ways to
have a baby too But we're a g rated site 00:02:51.243 --> [endTime] so we're just just saying moving on Oh
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