Vellum Venom: Daylight Opening (DLO), Defined

With reader feedback always on my mind, perhaps an overview of commonly used terms in the car design trade is needed. Let’s define the Daylight Opening (DLO) and dig into one of the more confusing terms in a car designer’s handbook.

Daylight Opening likely originated from architecture, as the first designers (a.k.a. coachbuilders) applied such techniques to create bespoke passenger cabins elegantly resting atop a rolling chassis. While most coachbuilt efforts lacked the brilliant integration of the 1937 Delage D8 120 S Pourtout Aero Coupe (pictured above), the marching of time ensured even the entry-level Geely HQ has an integrated form: therefore our DLO definition needed a re-think.

Simply stated, the Automotive Definition of Daylight Opening is the collection of glass in a car body, discussed as a single element from the front, back or side.

The US DOT provides mind-numbingly complex more precise definitions for body side and the front/back DLO, but we focus on the side as it’s a far more expressive place for a designer’s touch. Well, 1996-1999 Ford Taurus notwithstanding, the body side is usually the most expressive.

So today, DLO is commonly referenced when describing the large swath of glass down the body side, between the first and the last roof pillar. An appealing DLO makes your eye move quickly from front to back, complementing other styling elements, and visually speeding up the roof so it completes a vehicle’s sleek styling.

In the case of the BMW X2, the DLO does a fantastic job transitioning the muscular body side into its short rear glass and chiseled posterior.

Too bad the X2 has a black plastic cheater panel at the end of the DLO, giving the false appearance of a longer, sleeker glass element. (My not-so-loving nickname for such phenomena, DLO FAIL, requires a separate post.)

That said, never forget that doing the opposite, making the DLO more upright and architectural often projects the right image for a design, with no need for cheater panels: take the Land Rover Discovery.

The Disco’s double window treatment between the C and D-pillars is certainly an extreme example, not to mention recent changes to roof-crush standards (and head-curtain airbags) permanently altered our landscape: it’s likely stomped out a fair bit of DLO creativity.

Take the sedan’s DLO: Rolls Royce Phantom aside, most sedans now sport fastback rooflines worthy of Steve McQueen and their DLOs are forced to match, even when the aforementioned cheater panels are needed to make it work. Perhaps that will change over time!

[Images: BMW]