Rigid pipe is described by inner diameter (so 3/4" pipe is about 7/8" outside). Tubing ("soft" copper) is described by outer diameter. Tubing is usually connected by compression fittings, but you can solder it with the proper fittings.

See also How can I connect to copper pipe which is slightly oversized?.

Answer from isherwood on Stack Exchange
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Ferguson
ferguson.com › product › 3 › 4-in.-copper-coupling-with-dimple-stop-(7 › 8-in.-od)-ccf › 10045.html
3/4 in. Copper Coupling with Dimple Stop (7/8 in. OD) | Ferguson
Fitting Sizes: 3/4 in · Fitting Type: Coupling · IAPMO Listed: No · Length: 1-59/100 in · Material: Wrot Copper · Material Type: C12200 · NSF: NSF 372, NSF 61 · NSF Certified: Yes · Origin: Domestic · Pressure - Maximum: 700 psi · ...
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The Home Depot
homedepot.com › p › Everbilt-3-4-in-Copper-Pressure-Slip-Coupling-Fitting-C601HD34 › 100346736
Everbilt 3/4 in. Copper Pressure Slip Coupling Fitting C601HD34 - The Home Depot
Everbilt 3/4 in. Copper Pressure Slip Coupling Fitting C601HD34
This 3/4 in. pressure slip coupling is ideal for air conditioning and refrigeration and potable water applications. Constructed with top-quality copper, this coupling connects or repairs two lengths of tube and is commonly used aboveground for water supply piping. It features a solder cup and is furnished without a center stop. Its dezincification-resistant design protects the adaptor's mechanical strength.
(4.6)
Price   $2.96
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Amazon
amazon.com › Straight-Coupling-Fittings-Plumbing-HVAC › dp › B08BTW57SW
Amazon.com: Copper Fitting 3/4 Inch (HVAC) 5/8 Inch (Plumbing) - Straight Coupling With Rolled Tube Stop & HVAC - 99.9% Pure - 5 Pack : Industrial & Scientific
Copper Fitting HVACR OD 3/4" (HVAC Outer Dimension) 5/8" (Plumbing Inner Dimension) - Copper 90 Degree Elbow Fitting Connector with 2 solder cups & ACR - 99.9% Pure Copper - 5 Pack ...
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Ferguson
ferguson.com › product › 3 › 4-in.-copper-coupling-(clean-bagged,-7 › 8-in.-od)-cbccf › 301942.html
3/4 in. Copper Coupling (Clean & Bagged, 7/8 in. OD) | Ferguson
Fitting Sizes: 3/4 in · Fitting Type: Coupling · Length: 1-36/61 in · Material: Wrot Copper · Maximum Temperature: 400 · NSF: NSF 61 · NSF Certified: Yes · Pressure - Maximum: 582 psi · Proposition 65: WARNING: Cancer and Reproductive ...
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COPPERTUBINGSALES
coppertubingsales.com › collections › copper coupling w/stop › 3/4 copper coupling with stop (7/8 od)(copper x copper)
3/4 Copper Coupling with Stop (7/8 OD)(Copper x Copper)
3/4 Copper Coupling with Stop (7/8 OD)(Copper x Copper)
Please Note: Sizes listed refer to the pipe dimensions the fitting is compatible with . They represent the outer diameter (OD) or nominal (ID)pipe size. Copper Couplings with Stop ( Copper to Copper)  · Used in residential and commercial systems such as potable water, air conditioning and refrigeration. · *Cleaned and bagged options for Medical Gas applications are available , customers should call for pricing these options . Wrot Copper Couplings feature a stop for connecting two copper pipes or tubes together. Manufactured in an ISO 9001 certified facility, and backed by a 50-year limited w
Price   $0.82
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Lowe's
lowes.com › plumbing › pipe & fittings › copper pipe & fittings
Streamline 3/4-in Copper Rolled-Stop Coupling W 01034L at Lowes.com
Shop Streamline 3/4-in Copper Rolled-Stop Coupling W 01034L in the Copper Pipe & Fittings department at Lowes.com
Published   February 13, 2024
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The Home Depot
homedepot.com › p › Everbilt-3-4-in-Copper-Pressure-Cup-x-Cup-Coupling-with-Stop-Fitting-C600HD34 › 100343588
Everbilt 3/4 in. Copper Pressure Cup x Cup Coupling with Stop Fitting C600HD34 - The Home Depot
Everbilt 3/4 in. Copper Pressure Cup x Cup Coupling with Stop Fitting C600HD34
This 3/4 in. pressure cup x cup coupling is ideal for air conditioning and refrigeration and potable water applications. Constructed with copper, this coupling connects straight lengths of tube and is commonly used aboveground for water supply piping. It features a solder cup and a dimple stop. This adaptor needs pressure to maintain flow and has a 582 psi, maximum working pressure. Its dezincification-resistant design protects the adaptor's mechanical strength.
(4.7)
Price   $1.82
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Outdoorfurnacesupply
outdoorfurnacesupply.com › 3-4-copper-coupling-less-stop-c-x-c.html
3/4" Copper Coupling Less Stop [C x C] - OutdoorFurnaceSupply.com
3/4" Copper Coupling Less Stop [C x C]
3/4" Copper Coupling Less Stop [C x C] Skip to the end of the images gallery · Skip to the beginning of the images gallery · Be the first to review this product · $0.92 · Availability: Out of stock · SKU · CF3/4LS · Ask a Question about ...
Price   $0.92
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Hartville Hardware
hartvillehardware.com › product › sku-442220 › copper-sweat-fittings
NIBCO 3/4 x 3/4 Copper Coupling without Stop | Hartville Hardware
NIBCO 3/4 x 3/4 Copper Coupling without Stop
NIBCO 3/4" x 3/4" Copper Coupling without Stop is a versatile copper fitting used for connecting two sections of copper pipe. It is designed without a stop, which means it allows a seamless connection without any obstructions or interruptions within the coupling. This coupling is ideal for a wide range of applications, including plumbing, heating, and other copper pipe installations. It's compatible with copper tubing and can be easily joined using a sweat/solder method, ensuring a secure and watertight connection. Whether you're working on new installations or repairs, this copper coupling is
Price   $2.42
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Amazon
amazon.com › 3-4-copper-union › s
Amazon.com: 3/4 Copper Union
PROCURU 2-Pack 3/4-Inch Copper Union CxC Sweat Connection, Made in USA, Certified Safe for Drinking Water (CWUN0034-2P) · EZ-FLUID Plumbing 3/4" C X FIP LF Brass Copper Pipe Union C x Female Iron Threaded (Sweat Sockets X FIP) Connection Copper Fitting,Straight (1 Pack) · ChillWaves Brass ...
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Budget Heating & Air Conditioning
budgetheating.com › home › quick selector › supplies › copper › fittings / accessories › copper fitting 3/4 coupling
Copper 37175 | Copper Fitting 3/4" Coupling
Copper Fitting 3/4 Coupling
Copper Fitting 3/4" Coupling used to connect 3/4" diameter copper tubing. For use with Refrigeration, Ventilating, and Heating and Air Conditioning system installations as well as many other common uses.
Price   $4.25
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MSC Direct
mscdirect.com › product › details › 09347485
B&K Mueller - Copper Pipe Coupling: 3/4" Fitting, C x C, Presoldered | MSC Industrial Supply Co.
Copper Pipe Coupling: 3/4" Fitting, C x C, Presoldered
Find Copper Pipe Coupling: 3/4" Fitting, C x C, Presoldered at MSC Industrial Supply, serving the metalworking, safety, and MRO industries for over 75 years
Price   $2.66
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eBay
ebay.com › business & industrial › hydraulics, pneumatics, pumps & plumbing › fittings & adapters › other fittings & adapters › 3 4 copper fittings
3 4 Copper Fittings for sale | eBay
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Amazon
amazon.com › 3-4-copper-fittings › s
Amazon.com: 3/4 Copper Fittings
EZ-FLUID Plumbing 3/4" C X C Copper 45 Degree Elbow,LF Short Turn Pressure Copper Fittings with Sweat Solder Connection for Residential,Commercial. (10 Packs) · WHK 10 PackS 3/4" Press Propress x 3/4" Copper coupling Adater Fittings Connect From Cooper to Cooper Pipe(Lead-Free,No Stop) · ...
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COPPERTUBINGSALES
coppertubingsales.com › collections › copper coupling no stop
Copper Coupling with No Stop — COPPERTUBINGSALES
View full details · Original price $0.48 - Original price $0.48 · Original price $0.48 · $0.55 · $0.55 - $0.55 · Current price $0.55 · | / Save 0% Save % Quick shop · Add to cart · Compare · Save 0% Save % 5/8 Copper Coupling (3/4 OD)with ...