Edit: United Airlines & Lufthansa flights. Which baggage rules for personal item and carry-ons?

It's simple.....you follow the rules of whichever airline's name is on the metal.

Meaning...if Lufthansa's flying the plane, you follow their rules. If United is flying the plane you follow their rules.

If I'm understanding correctly, you are flying a United plane from the US to a connecting city, and a Lufthansa flight from there to Spain.

If that's the case, your checked baggage policy (pricing, size and weight) will be United's, all the way to your destination.

Your carry-on luggage is subject to whoever's metal it is (i.e., the name on the side of the plane, the carrier flying the plane.) Because it is their physical luggage bins and your luggage has to fit in those. It doesn't much matter if your luggage would fit in United's bins, if you're flying on a Lufthansa plane.

I have this issue all the time with my United-Lufthansa itineraries. My carry-on luggage, which might be ok with the less restrictive United policies on my originating flight, may not meet the Lufthansa policies for my connecting flight. So I always have to make a choice: check my luggage from the start with United or gate check anything that exceeds Lufthansa limits in my connecting city.

On your return flight, most likely you will start on a Lufthansa plane, so their carry-on rules will apply for your 1st flight. When you reach your connecting city, you'll be subject to the carry-on rules of whichever company is flying the plane for your international flight.

That’s a bit confusing to follow. If your bags aren’t within Lufthansa’s requirements for carry-ons then yes, you’re going to need to check.

Easy. Borrow a bag from a friend that fits the Lufthansa requirements. No need to buy a new bag.

Agree to Frank II and like to add that when traveling by plane from an airport in the EU and it comes to packing and boarding of carry-ons you should know and follow these security requirements;
https://europa.eu/youreurope/citizens/travel/carry/luggage-restrictions/index_en.htm

Your reservation should show what the baggage rules are for your ticket.

My experience is slightly different, from purely a practical standpoint.

Leaving the US, on a US carrier, I follow their rules. If I connect in Europe to a partner airline, I don't worry about it. Unless it is a very small airplane, they are not going to weigh or measure your carry-on bag, if it is obviously way over, they may gate check it for you. Your checked bags will not be prevented from going through to the final destination.

Coming back though, if my first flight is on the European partner, then those are the rules I follow, and likely will apply.

That may very well be what the Lufthansa rep was referring to as "Originating" carrier, the airline that you check in with, either your flight from the US, or the first carrier on your flight home. So coming back, if you are flying Lufthansa to connect to a United flight, follow Lufthansa's rules.

We didn't fly United, but Delta to London, returning by way of CDG Paris to Atlanta. We experienced the issue of the carryon rule for flights within Europe.
The carryon rule include the briefcase that I carried as well as a carryon. I exceeded the weight limit and had to check my carryon, costing me 40 GBP more.
In the USA, it wouldn't have been a problem.

If I connect in Europe to a partner airline, I don't worry about it. Unless it is a very small airplane, they are not going to weigh or measure your carry-on bag

That's like saying "I drove at 120mph and didn't get a speeding ticket therefore you won't get a speeding ticket if you do that. Until you do.

When you connect at a European airport, it is up to the gate agent to decide if a bag gets on or not. I've seen them ask everyone to put their bag in the sizers for 150+ seat planes. A lot will have to do with how full the plane is and what type of ticket you have.

Lufthansa can be very strict. On my last flight they didn't allow any wheeled bags onto the plane at all. We were told this at the gate and had to have our bags tagged as gate checked. We could take our bags to the plane but then had to gate check them. Almost all the overheads were empty.

The bags were returned to us as we departed the aircraft. If someone tried to take a rolling bag on board, the flight attendant turned them around to leave it at the bottom of the stairs.

How much over is your bag? Is it size or weight?

Lufthansa can be very strict. On my last flight they didn't allow any wheeled bags onto the plane at all.

I’ve seen similar on Lufthansa. Folks having to check their otherwise compliant rollaboards because their personal items were too big. Picture backpack book bags pretty well stuffed.

Really the best advice is to follow the most restrictive rules of the airlines you fly.

Appreciate all your feedback. Sounds like I will just pack according to Lufthansa guidelines regardless of what Lufthansa Rep relayed to me about the originating carrier setting rules for carry-on and personal item. Better safe. Thanks!

Yep. We flew Lufthansa to Spain 2 years ago. Our carry-on weight limit was 8kg. Air Canada code shared the same flight and they have no weight limit (you just have to be able to lift the bag into the overhead compartment on your own). In the waiting area, we could see some Air Canada passengers who had carry-on bags that probably weighed 15kg. It is what it is. We could have paid more for Air Canada if that was important to us.