Tension is the most important element of knitting. It is the building block of all knitting instructions.
What is tension? Tension for knitting is the number of stitches and rows that an average knitter will achieve to given unit of measurements, using specific yarn, pattern and needles. It is used to translate measurements into number of stitches and rows to be worked.
As a guide, yarn manufactures usually recommend a standard tension for each quality they produce. Their recommendations are based on best result achieved from test samples as to texture of fabric in terms of density (not too loose or too tight) and feel (not too floppy or too firm).
Practically every knitting instruction will state a tension at which specific items were worked out, except items like toys, where the finished size is of not such importance.
The ultimate tension decision is made by designer of the pattern. Designers decide what kind of fabric and stitch suits their design best after knitting a few swatches for best suitability. Tight and firm texture would be considered for coats, toys and household items. Loose and open would suits items like shawl or throws.
Needle size is decided on when required texture of the fabric is achieved. Once the number of stitches and rows to 10cm is established, the whole pattern is then calculated accordingly.
If your tension is not the same as quoted in the instructions, then your complete garment will be either bigger or smaller than measurements stated. The garment may look disproportional and feel too floppy or too firm. This may also affect the amount of yarn needed for such project.
Therefore, before commencing specific project, it is necessary to check your tension, even if the yarn you are going to use is well known to you. To do this, knit a swatch about 13cm square in pattern, yarn and needles as stated. Stretch swatch in each directions and allow to rest. Mark 10cm square in middle of test piece and count stitches and rows between markers:
If the number of stitches and rows is less than quoted, knit another swatch, using smaller needles. If the number of stitches and rows is greater, use larger needles. You may need to knit samples a few times until stated tension is achieved.
It is more important to obtain the right number of stitches than rows. This may be a problem when knitting to given number of rows, but you can easily compensate for this by adding or taking away rows during knitting. For example: If your row tension on DK yarn is 30 rows than standard 28 rows, then add 2 more rows for every 10cm knitted in length or if the pattern states to work 56 rows, you would have to work 60 rows.
For best result, you must obtain the tension quoted in the instructions. It is worth investing a little bit of time into knitting test pieces to avoid disappointments later.
You can simply keep your samples for unforeseen accidental repairs or collect them and when you have lots, join them into blanket or similar project.