What is the dress code for visiting churches?

What is the appropirate dressing for visiting churches in Florence? Is it true I have to cover my arms when I visit the Duomo?

When visiting churches in the summer months, both men and women have to have at least their shoulders and knees covered; so even when it's 40°C outside, no skimpy tops, spaghetti straps or bare midriffs are allowed inside. Shorts and skirts should be knee length to cover knees. Sleeveless or short-sleeved shirts are fine. If in doubt, carry a light shawl or long sleeve top to wrap around you when you go inside. Caps and hats should not be worn inside either.

While many of the smaller churches do not have the personnel to check to make sure every visitor is dressed appropriately, it is respectful to dress accordingly. For the Duomo, Florence's most visited church, there are staff checking everyone's dress, and for those who have accidentally or unknowingly come without appropriate clothing, they can supply a disposable cloth to cover shoulders, knees or other bare body parts so that you can enter the cathedral. At other times, without the appropriate dress you may be asked to leave.