Before I was overtaken by creeping Deadline Disease, I promised three things - some general principles on translating flat knit patterns to kntting in the round; a list of increase and decrease stitch equivalents for right and wrong side rows; and why someone would want to knit either way. That's a bit much for one day.  Even for long-winded me.  Let's start with the most subjective - why one would choose one style over the other.

To start off with I'l make some bold statements that kntting historians are welcome to debate.  Knitting garments in the round was far more prevalent during most of knitting's history.  Think of socks, hats, and traditional sweaters - most of them were worked that way.  Seamed knitting (knitting in the flat) appears to have taken off around the time that knitting made the transition from traditonal garment to fashion wear - roughly about the same time when written patterns became more prevalent.  I believe it did so in order to conform better to fashionable clothing's styles, tailoring and fit, and to allow greater reproduceability of results among an audience already familiar with sewn garment methods.

Why knit in the round?

• Simple, quick garment construction
• Simple pattern writing for boxy shapes
• Minimizes purling on stockinette pieces
• Eliminates (or minimizes) seaming
• Takes advantage of knitting's inherent elasticity for fit
• Easier production of stranded colorwork
• Easier production of texture and lace patterns that require increase/decrease manipulations on every row
• Ideology (there's a strong lobby of knitting purists that advocate it as the most natural or historically-connected way to knit)
• Fewer pieces to lose or match in gauge
• Flashing (color stacking of space dyed yarns) works better
• Easier to ravel back and add length or replace worn sections.
• If using one circ, fewer needles to use than knitting in the flat

Why knit in the flat?

• More complex and tailored garment shapes can be achieved
• Pattern writing for complex shaping is simpler for designers trained in garment construction theory
• Easier to adjust tif you need to produce complex garments in multiple sizes
• Seams can add structure and strength to a piece
• Smaller individual units can be more convenient to produce
• Intarsia colorwork is easier to do
  
• Eliminates cast on row join challenge, especially on fine gauge garments that employ large stitch counts
• Eliminates the at-join color jog problem (to be specific it substitutes a match stripes at seam problem)
• Can be easier to measure garments in progress to determine compliance with required dimensions
• In flight modification for fit can be simpler because problems can be spotted after one piece is made, and the entire garment does not need to be ripped back to make adjustments
• Easier to add width while the garment is in progress
• No scary DPNs or circs needed.  Fewer needles to loose compared to DPNs, no DPN juncture ladder problems
• Straight needles are less expensive than circs, and multiple lengths needn't be purchased.  No need to have multiple diameters of the same size needle on hand to accommodate tubes of various diameters (required unless many DPNs, a two-circ or the oversized circ method is used)
• Easier to block pieces before final finishing stage.

I've probably left the reason why you chose one method or the other off these lists.  Feel free to add it in a comment.

Each method has its own strengths and shortcomings.  Each has styles for which it is particularly suited.  And each can be manipulated to do most of what the other style is better suited to do.  You can make faux seams on something knitted in the round. You can do stranded colorwork on something knitted in the flat.  You can add shaping to an in the round piece through planned and judicious use of increases and decreases to mimic the fabric manipulations of darts or tailored seaming. 

n many cases there's logic in the choice of one method over another.  Dale's Norwegian stranded pieces are perfectly suited for knitting in the round.  They employ strategies like steeking to place shaped collars or introduce other construction features.  I'm sure people have done it, but I wouldn't want to translate one of them for knitting in the flat - there's nothing to be gained by doing so.  Complex tailoring like this from a discontinued Berroco pattern would be a headache to render in circular knitting.  BUT logic doesn't always prevail.  I have seen commercial patterns for stranded sweaters that ARE knitted in the flat.  I'd take a hard look at them to see if I could produce them in the round.   Likewise, I've seen all sorts of contortions and cutting done to circular knit patterns in the name of making them less boxy. Again I'd have to take a closer look to see if using an alternate approach was better.

While you're far more likely to see in-the-round direction today than you were 25 years ago when I started knitting, you'll still find that many fashion oriented magazines and yarn manufacturers booklets offer up more patterns for knitting in the flat than they do the other method.  I'm thinking pubs like Vogue Knitting, Adriafil's Dritto& Rovescio and most of the modern European books, plus the old Phildar, Aarlan, and Pingouin books. 

As in so many things, ideology does play a part.  You can find books written in the 1940s through 1960s that sniff at knitting in the round, calling it "peasant work" or noting it in passing as the dreary ancestor of  more modern applications.  And you can find books written by knitting revivalists that excoriate the torture of imposing tailored seamed construction on a medium that has so many virtues in its most simple form.  I'm dogma-agnostic.  I use whichever method is best for me to produce the project at hand.  Which brings me to the real reason why I think patterns are written one way or the other:

• It's what the pattern's author/designer is most comfortable with

Overall though, the motivation to change something from flat to circular knitting is far more common than yesterday's case.  Anything with a rectangular construction and minimal shaping  is a natural - especially sweaters with drop shoulders.  The exceptions might be sweaters in yarns that are prone to biasing or stretching, or ones in particularly flimsy yarns or knit in very open textures.  In those cases the structural integrity imposed by firm seams might be crucial to garment drape and longevity.  I'll look at this in more detail in the next overly long post.