Why Holes Appear at the Underarm
The underarm is the most structurally complex join in a seamless sweater, and small holes there are so common that many experienced knitters accept them as inevitable — until they learn the right technique. Understanding why they form is the first step to fixing or preventing them.
When you divide a top-down sweater at the underarms, you place the sleeve stitches on hold and cast on new stitches to bridge the gap between body and sleeve. Later, when you work the sleeves, you pick up those cast-on stitches from the underside. The problem lies at the four corners of this join: the point where the held stitches meet the newly cast-on stitches is not a clean seam — it's a junction between stitches formed in different directions, and that junction leaves a gap.
On a bottom-up sweater worked flat, the same issue appears when seaming: the corners of the underarm seam are the hardest points to close perfectly. Even with good mattress stitch technique, a tiny hole at each corner is common.
Fix 1: Closing a Hole After the Fact with Duplicate Stitch
If your sweater is already finished and you have small holes at the underarms, duplicate stitch is your repair tool. This technique literally retraces the path of an existing stitch using a tapestry needle and matching yarn, covering the gap without any knitting.
How to do it:
- Thread a 30 cm length of matching yarn onto a tapestry needle — not the sewing kind, a blunt-tipped tapestry needle.
- Turn the sweater inside out so you're working from the wrong side of the underarm.
- Anchor your yarn by weaving it through two or three nearby stitches on the wrong side.
- Come up through the hole, and work small stitches that mirror the V-shape of the surrounding knit stitches. The goal is to fill the gap by catching the yarn legs of adjacent stitches.
- Work in a spiral: close the hole itself first, then reinforce the surrounding two or three stitches by weaving the needle through them on the wrong side to prevent the hole from reopening under tension.
- Weave in the end securely on the wrong side. Do not make a knot — knots work loose under repeated washing.
This works best for holes up to about 3–4 mm. Larger holes may need to be addressed from both sides, or patched with a small duplicate-stitch panel.
Fix 2: Prevention During Construction
If you're still at the underarm stage of your sweater, prevention is far easier than repair. The standard technique is to pick up one extra stitch at each corner of the underarm join, then decrease those extra stitches away on the following round.
Step-by-step:
- When you reach the underarm on the body, place the sleeve stitches on waste yarn as normal.
- Using a backward loop (or knitted-on) cast-on, cast on your underarm stitches — typically 4–8, depending on your pattern. Don't cast on any extras yet.
- Continue around the body and finish the body. When you return for the sleeves, pick up the sleeve stitches from waste yarn onto your needle.
- Before joining to the underarm cast-on stitches, use your needle tip to pick up one stitch from the ladder of yarn at the corner — the small strand that bridges the gap. Place this picked-up stitch on the needle alongside the sleeve stitches. Do this at both corners (you're adding 2 extra stitches total).
- Work around to the first corner stitch and k2tog (knit it together with the adjacent sleeve stitch). Work the underarm cast-on stitches normally. At the second corner, ssk (slip, slip, knit) the corner stitch together with the adjacent sleeve stitch.
- Continue working the sleeve from here. The two decreases on this first round eliminate the extra corner stitches and close the holes simultaneously.
The Tight Underarm Trick
A complementary technique that makes a significant difference: when you reach the underarm stitches during your sleeve work, pull the working yarn firmly before and after these stitches. The cast-on edge at the underarm has a natural tendency to be looser than the body fabric around it — this is a structural issue with how cast-on stitches behave when picked up from the other direction. A firm tug on the yarn at this point takes up the slack and prevents the loose underarm look even if you don't have an actual hole.
Some knitters also cast on underarm stitches over a slightly smaller needle — one size down — to deliberately tighten this section. Cast on the underarm stitches with the smaller needle, then switch back to the working needle for the rest of the round.
Dealing with a Large Gap
If your hole is large enough that duplicate stitch won't cover it, you have two options:
Kitchener stitch repair: Thread the yarn around the two sides of the hole as if working Kitchener stitch (grafting), using the surrounding stitch legs as anchors. This creates a new set of stitches that bridges the gap seamlessly, matching the knit structure of the surrounding fabric. It requires patience and good lighting but produces an invisible repair.
Cut and re-pick: For a truly large or ugly hole, consider treating it as a surgical frog. Cut the yarn a few stitches back from the hole, carefully unravel to just above the hole, pick up the live stitches on a needle, and re-knit a small section of the underarm, grafting it back into the live stitches on the other side. This is advanced repair work, but it produces a factory-fresh result.
Final Check
After any underarm repair, block the sweater again — even a light steam from a damp cloth held close to the underarm area. Blocking relaxes the fibre and helps the repaired stitches blend into the surrounding fabric. What looks obvious when dry often becomes invisible after one good blocking.