An increase is simply adding a stitch to the knitting. Consider all of the ways you could create a new loop of yarn on the needle. Each way is likely to have been named and used by someone. If I could use only three methods (and I pretty much do only use these three), they would be M1L, M1R, and Yarn Over (YO). The first two are the most discreet and invisible, and match each other symmetrically for the fussy among us. YO is, by contrast, intentionally visible: it creates a decorative hole in the knitting, which is used a lot in lace. (It also, incidentally, makes a nice, stretchy little buttonhole.)
The sampler below contains both increases and decreases. Since it was knit from bottom to top, the increases are the "V" part of the diamond shapes. All of these increases are worked on the knit rows only (not the purl rows). The names M1A, M1T, KRL, KLL, M1B, and M1F are my own names, and not standard by any means. Unfortunately those increases do not have standard names to my knowledge, except perhaps M1L and M1R which is becoming common.
M1A
This doesn't match the right side
absolutely perfectly, but it is just fine for beginners.
M1A
(Make One Away) The easiest increase This is a good, all purpose increase,
as long as you don't mind a loose increase that can show a
small hole. When a pattern says to increase or to "make
one (M1)" you can use this (or one of the following M1
increases).
...make a backwards loop onto right needle, so that the working
yarn points away from you.
M1T
(Make One Towards) This matches M1A perfectly, and is
just as easy.
...make a backwards loop onto right needle so that the working
yarn points towards you.
M1A
M1R (Make One
Right) I think of it as: M1B (Make One Back) Matches M1F perfectly. A very neat,
tight, invisible increase. Can be tricky to work.
... From the back, lift loop between stitches with
left needle, knit into front of loop.
To work M1R on the Purl side:
M1L
(Make One Left) I think of it as: M1F (Make One Front) This creates the exact same stitch
as Make One Away, it just does it tighter and more invisibly.
It's as if you worked M1A on the previous row (very, very,
tightly), and then knitted it this row. Harder to work, but
my favorite increase for results. ... From the front, lift loop between stitches
with left needle, knit into back of loop.
To work M1L on the Purl side:
KRL (Knit Right Loop) ...insert right needle into right loop of stitch just
below next stitch; place it onto left needle and knit it;
then knit the stitch on needle.
KLL (Knit Left Loop) This pair of increases is ideal if
you ever need to do two increases side by side without any
stitches between them. Do KLL followed by KRL.
...Insert left needle into left loop of stitch two rows
below last completed stitch. Knit this stitch through
the back loop.
KFB
(work as for right side)
KFB (knit front and back)
also called a Bar Inrease, and k1f&b (knit
1 front & back)
...Knit a stitch, leaving stitch on left needle; knit into
the back loop of this stitch
To work this stitch on a purl row: ...Purl a stitch, leaving stitch on left needle; purl (up
into) the back loop of this stitch.
Moss Increase
(work as for right side)
Moss Increase
...Knit a stitch, leaving stitch on left needle; purl into
the front of this stitch.
Decorative Increase
(work as for right side)
Decorative Increase It's difficult to see in the picture,
but this increase creates a hole in the knitting, similar
to the Yarn Over, but smaller. If anyone knows of a better name for this increase, let me know.
...Knit into strand between stitches.
yo
(work as for right side)
yo (Yarn Over) ...Bring yarn forward and then
up over the right needle. That's it! When you've worked the
next stitch, the yo should have created an extra loop, or
stitch, between the last two stitches.