How to Read the Bug: Decoding Every Marking on Your Windshield

The bug is the small etched or screen-printed block in a lower corner of your glass, and it is a spec sheet hiding in plain sight. It tells you who manufactured the glass, which federal safety class it falls under, the manufacturer's part number, the country it was made in, and, through a pattern of dots next to a single digit, roughly when it left the plant.

We get asked about this constantly, usually by someone trying to work out whether the windshield in their car is the one that came in it. Here is how to read every piece of it, and what it honestly can and cannot tell you.

What is the bug on a windshield?

The bug, sometimes called the monogram or the trademark, is a permanent marking required on automotive safety glazing sold in the United States. It usually sits in the lower passenger corner of a windshield and in a lower corner of side and rear glass. On a windshield it is etched or ceramic-printed so it cannot be removed without destroying the glass, which is the point.

Every legal piece of automotive glass has one. If a piece of glass in your car has no bug at all, that is worth investigating, because it may not be automotive safety glazing.

What does AS1, AS2, and AS3 mean?

This is the most important line on the bug and the one most people skip.

• AS1 passes the full battery of impact and optical tests. It is the only rating permitted anywhere on a windshield above the AS1 line.
• AS2 does not pass all of the impact testing. It is used for side and rear glass, not windshields.
• AS3 is typically darker privacy glass, used in rear and quarter positions on trucks and SUVs.

If someone offers you a windshield marked AS2 for a car you intend to drive on the street, walk away. That is not a technicality, it is the difference between glass that is legal in that position and glass that is not.

What is the DOT number?

The DOT number identifies the registered manufacturer of the glass, not the car and not the vehicle manufacturer. Each glass producer is assigned a number by the Department of Transportation, and that number stays with them across every part they make.

This is the single most useful field if you are trying to figure out where a piece of glass actually came from. Two windshields that look identical can carry very different DOT numbers, and that tells you they came out of two different plants.

What is the M number and the part number?

The M number is the manufacturer's internal model or configuration number, and the longer alphanumeric string next to it is generally the part number a glass shop uses to look the piece up. If you are trying to source a replacement for an unusual car, photographing this string and emailing it to a glass supplier will get you further than describing the car.

You may also see a circled E with a number, which indicates the piece is certified for the European market, and a country of origin line telling you where it was manufactured.

How do you read the date code on glass?

Here is the part everyone actually came for. The date is usually encoded as a single digit for the year plus a group of dots for the month, and the dots are read as a countdown rather than a count up.

1. The digit is the year. A single digit, so you have to know roughly what decade the glass is from. A 9 on a 1969 car is almost certainly 1969, not 1979 or 1989.
2. Dot position tells you the half of the year. Dots printed before the year digit mean the first half of the year. Dots printed after the digit mean the second half.
3. Dot count counts down. Six dots is the first month of that half, and one dot is the last. So in the first half: six dots is January, five is February, four is March, three is April, two is May, one is June. In the second half the pattern restarts: six dots is July, five is August, and so on down to one dot for December.

Worth being straight with you: this convention is widely used but it is not perfectly universal. Different manufacturers and different eras have used variations, and some glass carries a printed month and year instead of dots. Treat the dot code as a strong indicator, not as a notarized document.

What can the date code actually tell you about your car?

The useful trick is comparing the glass date against the vehicle build date. Glass was typically installed within a few weeks to a few months of the car being assembled, so a windshield dated shortly before the car's build date is very likely the original piece. A windshield dated four years after the car was built is a replacement, full stop.

For a concours or numbers-matching car this matters to judges and to buyers. For a driver it is mostly a curiosity, though it is a fast way to sanity check a seller's claim that a car is unrestored.

What the date code cannot tell you: whether the glass is cracked, delaminating, pitted, or safe. A perfectly original 1969 windshield with a hazy sandblasted band across the driver's sightline is a worse windshield than a new one, no matter what the bug says.

What if your glass is date-correct but you want the modern look?

This is the real fork in the road, and there is no wrong answer, only a decision you should make deliberately.

If the car is an original survivor or a documented numbers-matching example, original glass has real value and pulling it out costs you something you cannot buy back. Keep it, and treat a chip repair as maintenance.

If the car is a restomod or a pro-touring build, the date code on the glass is not a factor in what the car is worth, and dated glass with a chrome trim ring around it is usually the last thing holding the build back visually. That is where flush-mount glass comes in.

Our flush-mount kits carry their own bug, DOT certified and built to FMVSS 205, laminated in front and tempered in the rear. New glass, legal glass, and no trim ring. If you want the full explanation of what that certification means, we wrote DOT certified glass for classic cars.

What if the glass you need is not made anymore?

It happens more than you would think, especially on lower-production and import classics. That is why we stock OEM-style replacement glass, including Pilkington pieces for cars nobody else supports. We wrote a whole piece on what to do when you hit that wall: nobody makes your windshield anymore.

Quick answers

Where is the bug on my windshield? Usually the lower passenger corner, occasionally the lower driver corner. It is small, so look in good light from outside the car.

My glass has no bug. Is that bad? On a windshield, yes, it is worth investigating. Legal automotive glazing carries a permanent marking. Unmarked glass may be aftermarket plastic or non-automotive.

Can the bug be removed? Not without ruining the glass, which is deliberate.

Does tint film cover the bug? Film goes on the inside, so the bug generally stays readable from outside.

Two pieces of my glass have different DOT numbers. Is something wrong? Not necessarily. It usually just means one piece has been replaced, or the automaker sourced from more than one supplier.

Is polycarbonate legal for a street windshield? No. It is not DOT approved and it scratches, hazes, and yellows. We covered it in polycarbonate vs laminated automotive glass.

Shop DOT-certified flush-mount glass, browse OEM replacement glass, or send us a photo of your bug and we will help you decode it.