The Klein bottle is a non-orientable mathematical surface formed by twisting a tube and attaching its two ends together. Like a Mobius strip, it is one-sided, and if you traveled along its surface you’d end up at the same point you started at but flipped upside-down. Here’s the wikipedia page (and a more readable introduction to the Kein bottle) if you want to learn more.

Klein bottles are perfect for crochet, and I was shocked there wasn’t already a pattern on Ravelry. So I decided to make one! I used these diagrams as a guide. This is a fun project (or gift) for science or math geeks or anyone interested in STEM-related crafting.

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This free pattern can be made using any yarn, hook size, or gauge. Also, Klein bottles can be any size or shape, so feel free to tweak these instructions if you want yours to be taller, wider, bigger, smaller, more or less curvature… you can use this one as a basis or reference and then go wild from there.

This pattern includes written instructions for every round, plus labeled photos showing how it should look at different steps. There is also a crochet stitch chart for the trickier part (the hole) if you prefer to work from a chart and a printer-friendly option without photos or notes.

This is a quick, easy pattern that should be very doable for beginners. You need to know how to work flat and in the round and some basic stitches:

• single crochet (sc)
• chain (ch)
• slip stitch (sl st)
• decrease (dec) (a.k.a. sc2tog)
• increase (inc) (a.k.a. 2 sc in a stitch)

For context, my Klein bottle was 4” (10 cm) tall and 3” (8cm) wide, made using bulky weight yarn and H hook (5.0 mm). I think a smaller one would make a great keychain!