What Do the Markings on My Windshield Mean? AS1, DOT, and M Numbers Decoded
Quick answer: That block of text in the corner of your windshield is called the bug (or monogram), and most of it is required by federal law. It tells you who made the glass, that it is laminated glass legal for use in a windshield, and which internal model the manufacturer built it to. It does not tell you the glass quality, the thickness, or when your car was built — three things nearly every article on this subject gets wrong.
Start here: find your bug
Look at the bottom corner of your windshield, usually on the passenger side, sometimes the driver's side. You'll see something like this, etched or screen-printed in white or black:
FUYAO · Laminated · AS1 · M854 · DOT-459 · SOLARTINT · DW1580
Or on an older car:
Carlite · AS1 · DOT 1777 · M848 · DW330
Or on an import, with an extra line:
E1 · 43R-000123 · AS1 · DOT 39
Same information, different order, different vendors. Once you know what each piece is, every windshield you look at for the rest of your life becomes readable. Here's the walkthrough.
The four things federal law requires
Windshields sold in the United States fall under Federal Motor Vehicle Safety Standard No. 205 — 49 CFR § 571.205 — which incorporates the industry standard ANSI/SAE Z26.1-1996 by reference.
NHTSA has spelled out what that standard's marking section requires. In the agency's own words, the glazing must carry:
"(1) the words 'American National Standard' or the characters 'AS,' (2) a number identifying the item of glazing, (3) a model number assigned by the manufacturer that identifies the type of construction of the glazing material, and (4) the manufacturer's distinctive designation or trademark."
Then S6.2 of the standard adds two more, in letters and numerals the same size as the rest: the symbol DOT, and a manufacturer's code mark that NHTSA assigns.
That's the whole legal core of the bug. Everything else you see is the manufacturer's own addition. Let's take them one at a time.
1. The trademark — who actually made your glass
The name or logo at the top is the prime glazing manufacturer's mark. Fuyao, Pilkington, Saint-Gobain Sekurit, AGC, Guardian, Vitro, XYG, Carlite.
A few worth knowing:
Carlite is Ford's brand. Carlex Glass America manufactures it, and per Carlex, "Carlite is the only brand recommended by Ford Motor Company for replacement in Ford or Lincoln vehicles." If you're looking at a Ford or Lincoln and the bug says Carlite, you're looking at OE-brand glass.
Fuyao is a Chinese manufacturer founded in 1987 that supplies Ford, GM, Tesla, Volkswagen, and Subaru as an original-equipment vendor, with U.S. plants in Moraine, Ohio and Mount Zion, Illinois. A Fuyao bug is not evidence of cheap aftermarket glass — plenty of vehicles roll off the assembly line with it.
An important wrinkle: the trademark on the glass and the company that made it aren't always the same. NHTSA has permitted distributors to put their own logo on glazing made by someone else, precisely because — as the agency put it — "NHTSA can easily and accurately identify the prime manufacturer from the DOT code mark, regardless of the distinctive designation or trademark that appears on the glazing."
Which means the logo is marketing. The DOT number is the real identifier.
2. AS1, AS2, AS3 — where that piece of glass is legally allowed to go
This is the marking people ask about most, and it's the most misunderstood. The AS number is not a quality grade. It's a location permission, tied to what the glass is made of and how much light it passes.
From NHTSA's interpretation of the standard:
AS1 — Laminated safety glass. "Required to be used in the windshield of every motor vehicle except a motorcycle and is acceptable for use anywhere in motor vehicles." Minimum 70% light transmittance.
AS2 — Laminated or tempered safety glass. "Acceptable for use anywhere in a motor vehicle except in the windshield." This is what most door glass and back glass carries.
AS3 — Glazing with "less than 70 percent light transmittance," acceptable "only in areas not requisite for driving visibility." Factory privacy glass in the rear of an SUV is the everyday example.
So: if you are looking at a windshield and the bug does not say AS1, something is wrong. That's the single most useful check a car owner can run on their own glass, and it takes four seconds.
The numbering also runs well past 3. Z26.1 defines item numbers up into the teens covering rigid plastics and glass-plastic materials — AS-4, AS-5, AS-14, AS-15 and others — which is why you'll spot unfamiliar AS numbers on RVs, buses, boats, and off-highway equipment rather than passenger cars.
3. The DOT number — the manufacturer's federal ID
DOT-459, DOT 1777, DOT 39. This is the manufacturer's code mark, assigned by NHTSA on written request to the agency's Office of Vehicle Safety Compliance. It exists so that if a glazing defect or recall surfaces, the agency can trace the piece back to the company that made it no matter whose logo is printed alongside.
Two things it is not:
It is not a quality ranking. A lower number doesn't mean better glass, and a higher number doesn't mean worse. It's a registry identifier, nothing more.
It is not a certification you can shop by. Every windshield legally sold for on-road use in the U.S. certifies to the same FMVSS 205 requirements. The DOT number says who, not how good.
What it's genuinely useful for is settling an argument. If a shop tells you they installed a particular brand and the DOT code on the glass traces to a different prime manufacturer, that's a real discrepancy worth asking about.
4. The M number — the one everyone decodes wrong
M854. M848. M241. You will find articles claiming the digits break down into thickness and tint — that the first two digits are the thickness in millimeters and the last digit encodes clear, green, bronze, or blue.
That is not what the M number is.
Under the federal standard, it's "a model number assigned by the manufacturer that identifies the type of construction of the glazing material." Key phrase: assigned by the manufacturer. It points at a specific construction that company built and tested — that interlayer, that glass composition, that build. It is internal to the manufacturer and is not standardized across brands.
Fuyao's M854 and Pilkington's M854 have nothing to do with each other. There is no universal decoder ring, and any page that hands you one is guessing.
The M number is still genuinely useful — it's how a manufacturer identifies which of its own constructions is in your car, and it matters when a vehicle has several windshield variants for the same model year. It just doesn't decode the way the internet says it does.
The markings federal law doesn't require
Everything below is optional, added by the manufacturer. It's often the most practically useful part of the bug.
The E-mark: European type approval
If your glass was approved for the European market, you'll see a circled E followed by a number, plus an approval number beginning with 43R — a reference to UN Regulation No. 43, the European equivalent of FMVSS 205.
The number after the E identifies the country whose authority granted the approval, not the manufacturer's home country. E1 is Germany; E43 is Japan. A German-approved windshield on an American car simply means the same part is homologated for both markets — common on European and Japanese vehicles.
You may also see a Roman numeral nearby indicating the glazing category under R43, and CCC if the part is also approved for the Chinese market.
DW and FW numbers: the part number your shop and insurer use
DW1580. FW2734. DD5501. These are NAGS numbers — National Auto Glass Specifications, published by Mitchell. NAGS is the catalog the entire U.S. auto glass trade runs on: parts, labor times, and the pricing insurers use to settle glass claims.
The prefix tells you the part:
DW / FW — Windshield (domestic / foreign vehicle)
DB / FB — Back glass
DD / FD — Door glass
DQ / FQ — Quarter glass
DV / FV — Vent glass
Not every windshield has its NAGS number printed on it — plenty don't. But when it's there, it's the single most useful marking for a consumer, because it's the number that appears on your quote and on your insurance claim. If the NAGS number on your invoice and the one etched in your glass don't match, ask why.
Feature words: SOLAR, ACOUSTIC, HUD, TINTED
SOLARTINT, Solar-E, Acoustic, Sekurit HUD, Tinted, SoundScreen — these are manufacturer descriptors for what the glass actually does:
Solar markings indicate solar-attenuating glass — an interlayer or coating that rejects infrared to keep the cabin cooler. Note that this doesn't get around the light transmittance rule: the driving-visibility area of a windshield still has to pass at least 70% visible light.
Acoustic indicates a sound-damping interlayer laminated between the glass plies. Vehicles that came with acoustic glass are noticeably louder with a non-acoustic replacement, and it's a real, audible downgrade people don't discover until the car is back together.
HUD indicates a wedge-shaped interlayer engineered for head-up display projection. A HUD car fitted with a non-HUD windshield produces a ghosted double image. This one is not subtle.
These are the markings worth checking before your glass is ordered, not after.
The date code — and what it actually tells you
Many manufacturers add a date of manufacture, commonly as a year digit with a row of dots. The prevailing convention: dots before the year mean the first half of the year, dots after mean the second half, counting down from six to one — so six dots before a "4" reads as January 2024, one dot before reads June, and dots after the number carry you July through December.
Three honest caveats:
FMVSS 205 does not require a date-of-manufacture marking at all. It's a manufacturer convention, not a federal one.
The format is not standardized. Different manufacturers use different schemes, and some use none.
It is the date the glass was made, not the date the car was built — and not the date the glass was installed.
That last point is the useful one. Glass is typically produced within a few months of the vehicle it goes into. So if you're looking at a 2019 vehicle and the windshield date code reads 2023, that windshield has almost certainly been replaced. If you're buying a used car, that's a five-second check that tells you something the seller may not have volunteered — worth pairing with a look at the urethane bead and the trim for signs of a rushed install.
But treat it as a lead, not proof. Some vehicles sat on a lot; some glass sat in a warehouse.
The AS-1 line: the marking that can get you a ticket in Texas
Look at the top of your windshield and you may find a small AS-1 notation on one or both sides, sometimes with a faint horizontal reference. That's the AS-1 line, and it's the legal boundary for the shade band at the top of the glass.
The federal standard sets where that line may sit — either per SAE J100 or per the geometry spelled out in S5.3.2, a plane inclined 7 degrees up from the vehicle's X axis through point V1. Below that line, the windshield has to hit 70% light transmittance. Above it, the shade band is allowed to be darker.
In Texas, this is written into the traffic code. Under Texas Transportation Code § 547.613, a windshield sunscreening device must not "extend downward beyond the AS-1 line or more than five inches from the top of the windshield, whichever is closer to the top of the windshield" — and where it is applied, it must leave at least 25% light transmission, no more than 25% luminous reflectance, and cannot be red, blue, or amber.
The Texas Department of Public Safety states the inspection rule plainly: "Sunscreening devices must be applied above the AS-1 line. If there is no AS-1 line, sunscreening devices must end five inches below the top of the windshield."
So the AS-1 marking on your glass isn't decorative. It's the line a Texas officer — or a state inspector — measures against. And if your replacement windshield doesn't carry the marking, the five-inch default is what applies to you.
Three things the bug does not tell you
It doesn't tell you the glass is OEM. OEM, OEE, and aftermarket glass can all carry a legitimate AS1 marking and a valid DOT code. The distinction is about the specification the part was built to and whether it carries the automaker's branding — not about federal compliance. Both can be perfectly legal.
It doesn't tell you the installation was any good. The single most common cause of a leaking, whistling, or unsafe windshield is not the glass — it's the urethane bond, the primer, the pinchweld prep, and whether the vehicle sat long enough to reach safe drive-away time. The bug says nothing about any of it.
It doesn't tell you your ADAS camera is aimed correctly. If your vehicle has a forward-facing camera behind the mirror, replacing the glass means that camera needs to be recalibrated — and no marking on the glass indicates whether that was done. That's a separate question with a separate answer, and we've written it up here.
What to actually do with all this
Next time you're standing at your car:
Check for AS1 on the windshield. Anything else in that position is a problem.
Read the trademark and the DOT number together. They should be consistent with what you were told you'd get.
Check the date code against your model year. A big gap means the glass has been replaced.
Look for feature words — Solar, Acoustic, HUD — and make sure your replacement quote matches them.
Compare the NAGS number on the glass to the one on your invoice, if it's printed.
Five checks, under a minute, and you'll know more about your windshield than most people who install them for a living.
Windshield Replacement in Fort Worth — With the Part Numbers Shown
Compass Auto Glass quotes by VIN, not by guess. Before we order, we tell you the exact glass going in your vehicle — brand, NAGS number, and whether it's solar, acoustic, or HUD-equipped — so you can walk out and read the bug yourself when we're done.
Same-day mobile service across DFW. We bill your insurance direct, recalibrate ADAS in-house, and back every install with a lifetime workmanship warranty.
817-229-1063
Serving Fort Worth, Arlington, Keller, Haltom City, North Richland Hills, and the DFW metroplex.
Sources: 49 CFR § 571.205 (FMVSS No. 205, Glazing Materials); ANSI/SAE Z26.1-1996 as incorporated by reference; NHTSA interpretation letters 8125a, nht74-2.34, and 2881o; Texas Transportation Code § 547.613; Texas DPS Window Tinting Standards; UN Regulation No. 43; Mitchell NAGS; Carlex Glass America.