A new slide in housing prices has begun in earnest, with averages in major cities across the country falling to their lowest point in many years.

Prices in 20 major metropolitan areas slid 1 percent in November from October, according to the Standard & Poor’s Case-Shiller Home Price Index released Tuesday. The index has fallen 1.6 percent from a year ago.

Nine of the 20 cities in the index sank in November to new lows for this economic cycle: Chicago; Las Vegas; Detroit; Atlanta; Seattle; Charlotte, N.C.; Miami; Tampa; Fla.; and Portland, Ore. Only a handful of places  essentially, California and the District of Columbia  went counter to the trend and had rising prices over the last year.

Whether the long-predicted double dip is looming or has already arrived is a quibble of semantics.

David M. Blitzer, chairman of S.& P.’s Index Committee, does not count a downturn as a double dip until it exceeds the previous low. The index is still 3.3 percent above the low it reached in April 2009. Mr. Blitzer thinks a double dip could be confirmed before spring.