Central Double Decreases

There are so many ways to remove stitches in knitting, and the end result is dramatically different when you're using the Central Double Decreases.  Take a look at each technique below.

A

B

Slip 2 sts, knit 1, pass both slipped sts over stitch just worked.  

Abbreviated as:  s2kp2   *or* sl 2, k1, p2sso

Slip 1 st, k2 sts together, pass slipped st over stitch just worked.

Abbreviated as:  s1k2p1  *or*  sl 1, k2tog, psso 

Central Double Decreases are often used with the front ribbing of V-neck sweaters, to shape the crown of a hat, and in lace knitting. 

When working a Central Double Decreases (CDD) as shown in image A, a column of knit stitches form along the center with the decreases neatly tucked under the center vertical line.  This is most often used when working V-necks or hats.  The effect looks unchallenged with minimal disruption to the pattern visually.  Hats will be seen from the top with straight lines pointing towards the center, instead of a pinwheel effect that often occurs with a series of regular left or right slanting decreases.  If you use this method in lace knitting, it will not be as attractive or eye catching, but it will serve the purpose of balancing out your stitch counts.  

When working the decrease as shown in image B, the center vertical line runs under the decrease stitches from both the left and right.  This is the one most often used in lace.  When spaced several rows apart, this gives the illusion that the center row weaves above and below the decreases.  The decrease forms a left slanting decrease which folds over onto the center line and the vertical line is broken.  This chopped appearance makes the eye pull to the decreases rather than to the center line and is most often used with working the decrease in connection with yarn overs as in lace knitting. 

Each decrease is attractive in its own right, but they look totally different.  For artistic purposes, you would decide whether you wanted a straight line or these little slanted tucks along the decrease line. 

The next time you're working a pattern that calls for a central double decrease, look at the abbreviation or glossary to find out the method being used.  You can use either method interchangeably without affecting the instructions or stitch count.  You will work up to the first stitch of the three stitches that you want to decrease into one stitch, then work the method you prefer.   
 
References:
Buss, Katharina.  Big Book of Knitting.  New York, NY:  Sterling Publishing Co., 2001.

Holladay, Arenda.  Blog “Double Decreases.”  Arendaholladay.com.  13JUL12. <http://www.arendaholladay.com/2012/07/it-has-been-unpleasant-week-which-began.html>;

Starmore, Alice.  Alice Starmore’s Book of Fair Isle Knitting.  Newton, CT:  The Taunton Press, 1988.