Here is Danjel's original post on the spec:
Sourceintellijel wrote:I knew there was a potential for some controversy with our choice of sizing for the 1U modules but we honestly had no ulterior motives behind this other than being the only practical way we could make them work with our design.
As far as following the lead of those who had come before us that is exactly what we did: we calculate sizes based on official 1U rack standards and Doepfer Eurorack measurements.
Given that 1U = 1.75" and a 1U tile panel is 1.7" that would only leave 0.025" for each rail lip (0.6mm!) and that is without accounting for size tolerances and extra spacing which is usually about +/-0.2mm
It was important for us that the 4U cases fit perfectly in a 4U rack space. This requirement drove all the other sizing.
We also looked at Doepfer 3U specs in terms of distance of panel holes relative to edge of 3U space, panel size vs. opening, rail thickness etc.
We also wanted to keep using the 5x2 pin headers for power so that the modules would be compatible with standard power supplies.
I actually think the idea on Tiles to use a mini header is very clever but it does not work for our modules since we have some that will be using the CV/gate bus and +5V rails.
In response to a recent post by pulplogic regarding Pulp Logic/Erthenvar 1U tiles fitting in a 1U rack space, Danjel clarified:
Sourceintellijel wrote:But that is because the rails you are using in this do not have any lip at all so they are atypical to the most commonly used rails (e.g. doepfer, tiptop etc.)
As stated in the other thread I made my calculations on the panel size to be the same ratio as the panels used by doepfer in the 3U size.
This is the only way it would work with our extrusions that have lips.
And in response to a question from DonaldCrunk about whether rails with lips are common enough to merit adopting a sizing different from that established by Pulp Logic/Erthenvar, James Husted of Synthwerks wrote:
Sourceersatzplanet wrote:The Vector rails indeed do not have a lip, which is there purely for cosmetic reasons. The standard Doepfer panel size will work with vector rails with no lip, Schroff rails with lips, Z-Rails with lips etc., etc. because they are sized for the HOLE SPACING, NOT the overall 3U size. If the original iU tiles made for the hole spacing of vector rails and rails with lips, they would work for both.
It is unfortunate that the originators of the 1U tiles didn't think of the possibility of the rails with lips being used. This means that buyers will always have to ask because now even the hole spacing is different for both versions.
NOBODY makes 3U modules that ONLY work for lipless Vector rails and not in the ones that have them. They are at least universal.
Long story short: Pulp Logic/Erthenvar 1U tiles use the full 1U of vertical space, but only work with Vector rails. Intellijel 1U modules use slightly less than 1U of vertical space, but work with all rails, including those with lips.
If anyone wants to add anything, or knows of any other pertinent posts by Danjel or others, please feel free to add them below.