If the new line will run through the cabinets where it can be seen when the cabinet doors are opened you may use compression fittings. If the line will be concealed under or behind the cabinets you must solder the joints.

Code prohibits compression joints in a concealed location.

Compression joints are easy to install and very reliable if installed correctly however you must be sure to use an approved tubing cutter to cut the copper.

Thanks Lazypup. The joint will be accessible via the sink base cabinet, so no problem there. Is a mini-hacksaw acceptable or should I just get the tubing cutter?

copper should never be cut by any means other than a tubing cutter.

When copper is cut with a tubing cutter the rotating action helps maintain the tubing in perfect roundness. When cut with a saw the ripping action of the saw will tend to distort the copper slightly out of round.

There is only 0.001" clearance between the tubing and a fitting. If the copper is even slightly out of round it is extremely difficult if not impossible to make up the joint.

My favorite cutter is a Superior Tool "Proline" mini cutter available at Lowes, HD and most hardware stores. Expect to pay about $10 for it, but i can assure you it is worth its weight in gold when you have to make cuts in tight places.

Imperial Eastman also makes a professional grade mini cutter but they are generally only found in HVAC supply stores.

When looking in your hardware you will also see a small red mini cutter for about $5. Take a tip from experience, unless you are in dire need of a fancy weight for your fishing line, don't waste your money.

Are compression fittings acceptable on
copper tubing to copper tubing
copper pipe to copper tubing
copper pipe to copper pipe?

Well, they have to be accessible. The fitting only connects ONE thing (tubing or pipe) to a fixture. You can get fixtures that go between all those things (unions, tees, elbows, etc..).