Listen carefully. There's something that ties together every iOS game from Scottish indie outfit Lucky Frame. From Pugs Luv Beats to Bad Hotel to the just-released Wave Trip.
You can't see it. The games look completely different. In one, you train pugs to run self-perpetuating radish farms. In another, you build weaponised hotels to kill crabs and yetis.
But, you can hear it. You see, all three games double up as procedural music machines, manufacturing new soundtracks on the fly in instant reaction to the way you play.
Every pug makes sounds as it bounces, every hotel turret has a signature tone, and every collectible in Wave Trip makes a distinctive noise. They coalesce to form a pleasing mish-mash of ambient noises and off-beat sounds.
At once, you're both playing the game and composing the soundtrack. To Yann Seznec, Lucky Frame's founder and director, however, the two actions are more similar than you might think.
Pugs Luv Beats
"I don't think it's any accident that the word 'play' is used for both games and music," Seznec explains. "You play games. You play music. Music is, essentially, a game of sorts, and I think there is an enormous amount of crossover between the two."
It's something we've seen before in games like Dreamcast headtrip Rez and submarine soundscape Electroplankton on DS. Two games that have inspired Lucky Frame.
"These games all have a really strong connection between user action and the audio generation, which is what encourages understanding and player interaction with the music," Seznec says.
It helps that Lucky Frame has music deeply ingrained in its company DNA. Seznec himself plays "blues and ragtime, and more crazy electronic music performance stuff", while company coder Jonathan Brodsky is a musical nut.
Their very first project together was an "8-bit Game Boy dance punk band called Handface". Lucky Frame's first project as a trio was "a music hack for Wii remotes".
Mixing games and music has been a decade-long passion for Lucky Frame, then.
"It's definitely a big part of our identity," Seznec says. "I tend to apply a very musical philosophy to almost everything we do."
Which brings us to Lucky Frame's latest musical contraption, Wave Trip.
It's styled like a side-scrolling shooter, but it's far more serene than Scramble. In Wave Trip, you're a geometric astronaut on a lazy rescue mission, and you must scoop up coloured gems to save your pals.
Every time you grab a gem, a new sound is momentarily added to the backing track. If you do well, you create a lush orchestra of beeps and boops. Do poorly, and your game sounds like a broken modem.
Wave Trip feels like the most natural blend of game and musical toy yet.
"Shmups lend themselves very easily to musical creation, because they are so linear," Seznec points out. "When you lay out a musical track along a timeline, it feels very similar to laying out a side-scrolling shooter."
Making your own Wave Trip level - and composing your own score - really hammers home the similarities between the two disciplines.
Lucky Frame's audio adventures have earned the studio two spots (in 2012 and 2013) at the IGF awards.
"The Pugs Luv Beats nomination took us completely by surprise, and it helped us really make a name for ourselves," Seznec says.
"By the time Bad Hotel launched, we had much more credibility, and I think everyone in the indie game community really understood what we were trying to do."
"I am so proud of the team and I am so humbled that our output is getting recognised alongside such masterpieces as Super Hexagon and Vesper.5."
But, not everyone is a fan.
In 2012, Apple rejected Lucky Frame's generative music app unVerse. It was deemed "not very useful", and apparently didn't "provide any lasting entertainment value".
It wasn't a huge setback, though.
"unVerse was always a fun side project for us, anyway," Seznec says. "It's not like we were depending on unVerse sales to continue."
It was, however, a formative experience for the team.
"It was an interesting case study in how different aspects of interaction are more valued than others - in particular, how sound is seen as not important."
It hasn't stopped Lucky Frame from making games for Apple's mobile devices, mind, and, indeed, thinking about launching games on other platforms.
"We would really like to start pushing to other platforms, and we've made a few moves to help that along," Seznec says.
In whichever app store that next game lands, you can be sure that it will have a strong musical focus. It's in Lucky Frame's genes, after all.
"At the moment, we're working on a two-player head-to-head Victorian duelling game," Seznec reveals. "It should have a procedural generated music system built in."
Yeah, we thought as much.