Men’s Underwear Fly Types: What You Must Know

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Many guys deal with their underwear fly on a daily – more than daily – basis. Other men—like me—don’t even consider the fly on our underwear because we don’t use it. The cliché question of Boxers vs. Briefs should be Fly or No Fly.

But for those who care about your underwear fly, have you stopped to really think about it? Or do you check to see if there is a fly and move on? If so, you may miss improving a crucial part of your underwear.

In this blog post, I review the main men’s underwear fly types and what to look for. I hope that you learn a bit about this often overlooked – yet vitally important for some guys – feature of men’s underwear.

What is the Purpose of the Men’s Underwear Fly?

I hope most men understand the purpose of the fly on their underwear. But if you don’t, or you aren’t a man and are curious about this unique feature of men’s underwear, I’ll explain.

Men pee using their penis. And most men – but not all – pee standing up. It is a hassle and a waste to have to lower your pants and your underwear just to pee. Thus, the fly provides easy access to your penis without having to remove your pants and underwear. The fly can also be used in sexual situations without having to undress.

Unlike a pouch, a fly is not required for men’s underwear to be comfortable. In fact, I prefer underwear without a fly, which I describe in more detail later on in this blog post. Furthermore, many men sit down to pee, as evidenced by this YouGov poll. If so, then you need not worry about your underwear fly and I recommend getting underwear without a fly (but with a pouch).

Men’s Underwear Fly vs Pouch

Don’t confuse the fly and the pouch on men’s underwear. The fly is specifically the slit on the pouch to provide interior access. The pouch supports and cradles the penis and balls. While interconnected, the two terms are not interchangeable. If you want to know about men’s pouches, read my complete Guide to Men’s Underwear Pouches here.

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Four Main Men’s Underwear Fly Types

Below are the four most common fly designs on men’s underwear. Note that the vertical fly is by far the most popular, although more and more men’s underwear don’t include flys at all. I don’t use a fly, and according to the Internet, only around 20% of men use one.

Vertical Fly/Keyhole Fly

When most guys – and women! – think of a men’s underwear fly, they imagine the standard vertical fly. As the most popular men’s underwear fly type today, the vertical fly is a common feature across various designs, brands, and styles. The design is simple: two pieces of fabric overlap in the pouch area. You put your right hand in between the two pieces of fabric, pull out your penis, and pee.

Pros:

• Ubiquitous: Because the vertical fly is so popular, you can be very picky with your underwear and still find a brand that makes it with a vertical fly. Your choices are almost endless.
• Simple Design: Because the design is so simple, there isn’t much variation between brands. This makes it easy to comparison shop and know what to expect.
• Right-Handed Universe: As right-handedness dominates humans, almost all vertical flys are designed for right-handed people (you enter from the right if you are looking down).

Cons:

• Tough for Left-handed Men: If you are left-handed, I am sorry. Even in the world of men’s underwear right-handedness reigns supreme. However, there is hope! Mack Weldon’s Silver HD underwear features a Dual-Sided Fly, allowing you to enter from either side.

Buy Now: Mack Weldon Silver HD Boxer Briefs with Dual-sided Fly

• Size Matters: Although the vertical fly design is simple, one part varies from brand to brand: the size of the actual opening. I don’t have a problem with most vertical flys I test. However, I can immediately feel the difference in size when using a smaller-than-usual fly. If you are large, you may sometimes get flys that are just too small to accommodate you comfortably.
• It is Awkward: I generally don’t use flys because I find the vertical fly awkward at public urinals. If you use the fly of your pants to access your penis, then navigating the vertical fly can be tough to manage in a small space. This is especially true if you wear tight pants or if the natural direction of your penis is to the right.

Horizontal Fly

The Horizontal Fly is a newer men’s underwear fly design. It is far less common than the vertical fly; I only have a handful of pairs with this fly design. However, I may like it a lot more than the vertical fly. It seems more natural to handle your penis, especially if your penis naturally rests in the up position.

Pros:

• More Natural Design: I find it easier to access my penis than the vertical fly. Other guys who have tried the Horizontal Fly seem to swear by it.
• Equal Treatment for Lefties: If you are left-handed, you won’t find the horizontal fly awkward, unlike the right-handed vertical fly.

Cons:

• Limited Selection: Not many brands have embraced the Horizontal Fly, so you’ll have to do some extra digging to find a horizontal fly with a pair you love. Of course, you can email me for recommendations: hello@mensunderwearguide.com
• Size Still Matters: My first Horizontal Fly pair of trunks had a small opening. However, the Tommy John trunks I bought later have a nice, large opening. So yes, the size of the opening still matters on the horizontal fly.

Last update on 2025-07-15 at 19:20 / Affiliate links / Images from Amazon Product Advertising API

No Fly

The lack of a fly is a type of men’s underwear fly! Most of my underwear does not have a fly; I pee by pulling down on my waistband and lifting my penis up and over. This is my preferred method, and I find it much easier than messing with a fly. It is also increasingly common today to see men’s underwear without a fly, even compared to just a decade ago.

Pros:

• More Selection: Nowadays, there are more men’s underwear options without a fly than previously. However, if you prefer to go over your waistband, then you can wear any type of men’s underwear without worrying about the fly design. The universe of men’s underwear is your oyster!
• Less Fabric: Horizontal and Vertical Flys rely on two pieces of overlapping fabric to prevent your penis from making unwanted appearances. Underwear without a fly do not need this extra fabric because there is no opening. This can result in a cooler and roomier pouch, but it depends on the design.
• Streamlined Look: I also like the look of men’s underwear without a pouch. It looks modern and sophisticated, especially compared to the vertical fly. This has nothing to do with comfort or utility, but for me, it is a benefit of having flyless underwear.

Cons:

• No Fly: This is obvious, but if you genuinely want a fly, then buying underwear with no fly is pointless.
• Waistband Size: Since you are peeing by pulling your waistband down, a larger waistband will make this process harder. Also, if your pants are tight, there may not be much room to reach up and grab your underwear waistband to pull down.
• Stuck with Your Choice: If you prefer peeing over your waistband, then you can wear underwear with a fly because it doesn’t matter. However, if you prefer using a fly and buy underwear without one, then you are stuck with your decision.

Last update on 2025-07-15 at 17:32 / Affiliate links / Images from Amazon Product Advertising API

Button Fly

The button fly is the least common men’s underwear fly design. When it is used, it is mostly used on men’s boxer shorts, but can be used on boxer briefs and trunks too. This is because the fabric of men’s boxers is thin and not fitted. Using a vertical fly design can make it relatively easy for your penis to come out, which may not be what you want if you are lounging around in boxers.

To help keep your penis inside, brands put a single button (sometimes two buttons) on the fly to keep it locked up. To use the fly, you unbutton the fly, and then it becomes a standard vertical fly. When done, you re-button the fly. This is my least favorite fly design by far. I think the whole process is just a hassle.

Pros:

• Security: If you wear boxers and are concerned about your penis flopping out, then having a button fly will provide you with a sense of security. Just make sure to use the button!
• Less Fabric: Button flys are generally used with flys that have thin material. The vertical fly uses a lot of material to provide overlap and keep the penis inside. If you design a fly without this excess material, it can be more comfortable but then there isn’t any material to keep your penis inside. Adding the button brings this security.
• Potentially Easier Access: Button flys tend to have the opening in the center of the pouch. This is different than vertical flys that have the opening to the far side of the pouch, or the horizontal fly that has it at the top. For the right guy, having the opening in the center might be easier. But for me, I just don’t like the button pouch.

Last update on 2025-07-15 at 18:49 / Affiliate links / Images from Amazon Product Advertising API

Cons:

• What a Hassle: I have worn underwear with a button fly, and I thought the buttoning and unbuttoning of the fly was a hassle. This is especially true if you are doing this at a public urinal in the tight space of your pants.
• Limited Selection: There is a limited selection of button-fly men’s underwear. Most button flys are on boxer shorts. I had a couple of American Eagle ribbed boxer briefs that came with a button fly, but they are uncommon.
• Don’t Lose the Button: You are out of luck if you lose the button because it comes loose through wear and tear.
• One vs Two Buttons: If you buy underwear with a button fly, consider whether there is one button or two buttons on the fly. Two buttons are more secure and prevent flopping, whereas one button is easier to unbutton for access.

Last update on 2025-07-15 at 19:20 / Affiliate links / Images from Amazon Product Advertising API

Alternative Fly Designs

There are a couple of less common fly designs that I want to highlight.

Separatec Dual Pouch Flap

I am a big fan of Separatec’s underwear, having previously written a full review of their Dual Pouch. Separatec’s claim to fame is their “Dual Pouch,” in which your penis is physically separated from the rest of your groin through a fabric sheath.

Because your penis is now in its separate fabric compartment, how do you pee? Separatec’s design includes a flap at the very end of the sheath. You lift the flap, and you pee. Think of it like flipping up the top of a Pez dispenser. Learn more about the Dual Pouch here.

When I wear Separatec, I am likelier to use the fly than other underwear. The reason is that if I want to go over my waistband, I have to pull out my penis from the sheath. When I am done, I have to insert it back in to use the underwear best. This adds an extra step and can be hard to do in a tight space like the fly of your pants. The flap is easy to use and makes the whole process simpler.

Pros:

• Simple to Use: This design is quite simple to use because you don’t have to move your penis around, unlike the vertical and horizontal fly designs. You only have to lift the flap at the end – like a cap for your penis – to gain access.
• No Need to Worry about Opening Size: One of my pet peeves with the vertical and horizontal fly designs is that if the opening slit is small, it makes it quite hard to use the fly. This isn’t an issue with the Separatec flap design because no positioning is required.

Cons:

• Only Found on Separatec Underwear: Since this is a proprietary design, you’ll only find this flap design on Separatec underwear and select “Dual Pouch” pairs of David Archy underwear.
• Requires Dual Pouch design: Most guys I recommend Separatec to love the Dual Pouch design (my husband is an exception). But if you don’t like the Dual Pouch design, then you are out of luck with this fly design, as it requires the Dual Pouch.

Y-Fronts

The classic Y-Front Fly design is outdated, having been replaced by the standard vertical fly. However, older generations may fondly reflect on the Y-Front fly and reminisce. If so, Y-Front briefs are still available, although selection is minimal.

For younger guys unfamiliar with the Y-Front fly, it is best described as a cross between the vertical and horizontal fly. It sits a lot lower on the pouch than other fly designs, and the opening slit is tilted at a 45-degree angle. Unless you specifically seek this design, stick to the other fly designs.

Last update on 2025-07-16 at 09:05 / Affiliate links / Images from Amazon Product Advertising API

My Recommendations

The type of underwear fly you prefer is really a matter of personal preference above all. But that doesn’t mean you should ignore the fly. Especially if you prefer using flys, explore both the vertical and horizontal fly designs. Seek out brands with larger openings – like Tommy John – if your current flys are too small for comfort. Or if you love wearing boxers but your penis flops out, get boxers with a button fly.

On the other hand, if you don’t use your fly, you don’t have to worry about this. However, I’d encourage you to buy underwear without a fly as I find it more comfortable and it looks better, in my opinion.

In the comments, let me know your thoughts on men’s underwear fly designs. If you need personalized recommendations, email me at hello@mensunderwearguide.com.

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