What is the difference between Single-Spring and Double-Spring rear derailleurs?

Rear derailleurs are designed to maintain chain tension regardless of which gear the chain is on. There are mainly two types: Single-Spring and Double-Spring.
Single-Spring derailleurs maintain chain tension using only the spring on the pulley cage rotation axis (P-tension).
Double-Spring derailleurs have an additional spring at the derailleur mount (B-tension), providing tension at two points.

Examples using Shimano mechanical 11-speed road and gravel rear derailleurs:

Single-Spring derailleur examples:

• RD-R7000 105
• RD-R8000 ULTEGRA
• RD-R9100 DURA-ACE
• RD-RX805 ULTEGRA RX
• RD-RX810 GRX
• RD-RX812 GRX

Double-Spring derailleur examples:

• RD-5800 105
• RD-6800 ULTEGRA
• RD-9000 DURA-ACE
• RD-U5000 METREA

If you are unsure which type of derailleur you have, please contact the store where you purchased it.