If opening jars becomes more difficult because of painful hands, or if climbing stairs produces pain in your knees, “arthritis” is often the first thing that comes to mind. The two most common forms of arthritis—osteoarthritis and rheumatoid arthritis—can cause similar aches and pains, but there are a few key differences between them. For example:

Onset. Osteoarthritis occurs when cartilage (tissue in your joints that cushions your bones) wears away. Pain occurs when bone rubs against bone. This type of arthritis pain tends to develop gradually and intermittently over several months or years.

Osteoarthritis is the most common type of arthritis affecting 27 million Americans. Many people believe it’s a crippling and inevitable part of growing old. But things are changing. Treatments are better, and plenty of people age well without much arthritis. If you have osteoarthritis, you can take steps to protect your joints, reduce discomfort, and improve mobility — all of which are detailed in this report. If you don’t have osteoarthritis, the report offers strategies for preventing it.

Rheumatoid arthritis, on the other hand, is an inflammatory condition in which your immune system attacks the tissues in your joints. It causes pain and stiffness that worsen over several weeks or a few months. And joint pain isn’t always the first sign of rheumatoid arthritis—sometimes it begins with “flu-like” symptoms of fatigue, fever, weakness, and minor joint aches.

Location. Both osteoarthritis and rheumatoid arthritis can affect the hands. However, osteoarthritis often affects the joint closest to the tip of the finger, whereas rheumatoid arthritis usually spares this joint. And while rheumatoid arthritis can appear in any joint, its most common targets are the hands, wrists, and feet.

Type of stiffness. People often describe vague muscle aches as “stiffness,” but when doctors talk about “stiffness,” they mean that a joint doesn’t move as easily as it should. Stiffness may be prominent even when joint pain is not.

Mild morning stiffness is common in osteoarthritis and often goes away after just a few minutes of activity. Sometimes people with osteoarthritis also notice the same type of stiffness during the day after resting the joint for an hour or so. In rheumatoid arthritis, however, morning stiffness doesn’t begin to improve for an hour or longer. Occasionally, prolonged joint stiffness in the morning is the first symptom of rheumatoid arthritis.