What to do
Check the exact allowance for your booking before you start packing. Look at the confirmation email or the manage booking page for the numbers that apply to your ticket. Measure the bag you plan to take, including wheels and handles. Weigh it if the airline states a limit. Keep the most valuable and the most needed items in the bag that will stay with you.

Three practical approaches
First approach is the standard cabin bag plus a small under-seat item. This is the most common setup on regular tickets. The bigger bag usually has to fit a size around 55 x 40 x 23 cm, though a few centimetres difference is sometimes tolerated. Weight limits often sit between 7 and 10 kg when they exist. The small item can be a handbag, laptop bag or small backpack that goes under the seat.

Second approach is the personal item only. Many basic fares allow just one small bag that must fit under the seat. Size is typically limited to something like 40 x 30 x 15 cm or similar. Weight is rarely checked strictly on these, but the bag still has to look small. This forces tighter packing and usually means checking the larger suitcase.

Third approach is travelling with a soft bag that can be compressed. Soft-sided bags are easier to squeeze into the sizer at the gate if the dimensions are borderline. They also tend to handle the weight distribution better. This does not change the official rules but it reduces the chance of a last-minute argument at boarding.

Cost
There is no direct cost for following the rules, but exceeding them can bring gate fees that range from 30 to 80 euros or dollars or force you to check the bag at the last moment for a higher price. Some tickets make the cabin bag free only if you pay for a higher fare. Knowing the exact limit in advance avoids those surprise charges.

Summary
Hand luggage rules are simpler when you ignore the general internet lists and look only at the numbers for your specific ticket. Measure the bag, respect the weight if one is stated, and keep essentials in the piece that stays with you. The three common setups cover almost every case: full cabin bag plus small item, personal item only, or a soft compressible bag. Check once before packing and the gate becomes far less stressful.