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If you have the premium Delta Reserve card, you can use the companion certificate for a Comfort+ seat. David Slotnick/Business Insider

• Four Delta credit cards come with a companion certificate each year when you renew on your account anniversary
• Delta companion certificates are essentially a buy-one-get-one coupon for domestic flights.
• When you book a round-trip ticket for yourself, you can bring a companion for just the cost of taxes and fees.
• Read Insider's guide to the best travel rewards credit cards.

Delta credit cards come with great perks for Delta flyers, like free checked bags and priority boarding on Delta flights, airport lounge access, and discounts on certain inflight purchases.

But one of the most valuable Delta benefits is the ability to bring a friend or family member with you for just the cost of taxes and fees. This comes in the form of an annual companion certificate — and four co-branded Amex Delta credit cards offer it — all with hugely increased limited-time bonuses:

• Delta SkyMiles® Platinum American Express Card — 80,000 bonus miles after you spend $4,000 in purchases on your new card in your first 6 months from account opening (offer ends 11/9/22)
• Delta SkyMiles® Platinum Business American Express Card — 85,000 bonus miles after spending $4,000 in purchases on your new card in your first 3 months of card membership (offer ends 11/9/22)
• Delta SkyMiles® Reserve American Express Card — 95,000 bonus miles after you spend $6,000 in purchases on your new card in your first 6 months from account opening (offer ends 11/9/22)
• Delta SkyMiles® Reserve Business American Express Card — 100,000 bonus miles after spending $6,000 in purchases on your new card in your first 3 months of card membership (offer ends 11/9/22)

It's possible to get well over $1,000 in travel from each of these bonuses. Read our post on how to use Delta miles for tips.

After your first year of holding one of these cards, you'll get a companion certificate. You'll receive another certificate each year you renew your card membership. Depending on how you use it, you could save hundreds — or even thousands — of dollars each year.

If you'll use this perk each year, you can easily offset your credit card annual fee. For example, the Delta SkyMiles® Reserve American Express Card charges a $550 annual fee. If you and a travel buddy wanted to fly from Atlanta to Seattle in business class, you'd pay $1,500 per ticket. But with the companion certificate, you could buy one ticket and bring your buddy along for just the cost of taxes and fees — effectively cutting your price per ticket in half.

Here's how this benefit works.

We're focused here on the rewards and perks that come with each card. These cards won't be worth it if you're paying interest or late fees. When using a credit card, it's important to pay your balance in full each month, make payments on time, and only spend what you can afford to pay.

Delta credit cards offer different companion certificates

The companion certificate is essentially a buy-one-get-one coupon. You'll only pay for the taxes and fees of your companion when flying Delta, Delta Connection, and Delta Shuttle flights.

Depending on which card you have, you can use the certificate for travel in economy, or in any class:

• Delta SkyMiles® Platinum American Express Card: Companion certificate valid for travel in economy ($250 annual fee)
• Delta SkyMiles® Platinum Business American Express Card: Companion certificate valid for travel in economy ($250 annual fee)
• American Express Delta SkyMiles® Reserve American Express Card: Companion certificate is valid for travel in economy, Delta Comfort+, or first class ($550 annual fee)
• Delta SkyMiles® Reserve Business American Express Card: Companion certificate is valid for travel in economy, Delta Comfort+, or first class ($550 annual fee)

As you can see, the Delta Reserve credit cards demand a significantly higher fee. That's because the cards also offer complimentary Delta Sky Club access and Amex Centurion lounge access when you're flying Delta.

If you're not sure which of these cards is best for you, check out our comparison of the Delta Platinum and Reserve cards to help decide.

You can't fly just anywhere with a Delta companion certificate

Delta companion certificates are only valid for flights within the continental US. In other words, you can't use them for international flights in premium cabins with lie-flat seats. You're relegated to standard domestic first-class recliner seats within the Lower 48.

Delta Companion certificates are eligible for upgrades based on the elite status of whichever traveler has higher Delta elite status.

To be clear, Hawaii, Alaska, and other US territories like Puerto Rico are not eligible destinations for your companion pass. However — if the address listed on your Delta SkyMiles account is in Alaska, Hawaii, Puerto Rico, or the US Virgin Islands, the certificate can be used for travel to the continental US.

Delta companion certificates only work with select fare classes

Holding a companion certificate doesn't mean you can book any flight within the contiguous US. You must pay attention to the specific "fare class" of each cabin. Fare classes are letters airlines use to categorize each seat on the plane. They determine things like price, ticket flexibility, eligibility to upgrade, etc.

You can attach a Delta companion certificate to any of the following fare classes:

• Economy: L, U, T, X, and V fares
• Comfort+: W, S (Delta Reserve cards only)
• First class: I, Z (Delta Reserve cards only)

In short, economy covers most of the cheaper and mid-priced coach fares, as well as some pricier ones. You shouldn't have much trouble finding eligible flights in economy as long as you're booking at least one month in advance.

You must use your eligible Delta card to pay

When reserving a companion certificate, you must pay with your Delta Platinum or Delta Reserve credit card to activate the benefit. You must also book both seats at the same time. 

You can't combine any web-special fares or eCoupons with your payment method. Also, you cannot use Delta miles in tandem with a Delta companion certificate. You must purchase your fare with your eligible card to qualify.

Amex Delta Platinum and Reserve cards bonuses

Right now, the Amex Delta Platinum and Delta Reserve cards have great welcome bonus offers (along with other Delta cards). Here's what you can earn:

• Delta SkyMiles® Platinum American Express Card — 80,000 bonus miles after you spend $4,000 in purchases on your new card in your first 6 months from account opening (offer ends 11/9/22)
• Delta SkyMiles® Platinum Business American Express Card — 85,000 bonus miles after spending $4,000 in purchases on your new card in your first 3 months of card membership (offer ends 11/9/22)
• Delta SkyMiles® Reserve American Express Card — 95,000 bonus miles after you spend $6,000 in purchases on your new card in your first 6 months from account opening (offer ends 11/9/22)
• Delta SkyMiles® Reserve Business American Express Card — 100,000 bonus miles after spending $6,000 in purchases on your new card in your first 3 months of card membership (offer ends 11/9/22)

Insider estimates that Delta miles are worth 1.1 cents each, on average. That means you'll receive potential value between $880 and $1,100 in flights, depending on which card you open.

Bottom line

The Delta companion certificate can be one of the most valuable ongoing credit card benefits in the travel credit card world.

Unfortunately, the amount you save will depend on the amount you spend. If you book a $1,000 seat for yourself, you can bring a friend for just the cost of taxes and fees, saving nearly $1,000.