The pattern of the holes of the filter cap seems wisely designed in function and esthetic manner. It is clear for me why there are (the round ones) holes on the top, flat side of the cap. The coffee needs some way out… 😃 But for what reason are also quadratic holes on the round, outer side of the cab inside the rabbets? Have these any special function? Or are they only for a nicer design and looks better? Well my suggestion at first was, it could be a kind of “Filter Bypass”. The filter paper itself will cover all round holes of the top flat side of the cap so no coffee can come out without filtration. But if the filter is in place, it is too thin to also cover the whole hole of the outer quadratic holes on the side of the cap. There is almost 1mm left so theoretically coffee and grinds could pass through it. Technically this gap is limited because the ring of the main tube will almost cover these gap completely if you screw up the cap on it. But I figured out, the gap will only nearly disappear if I give extreme force on the cap at screwing it up so I am at 100% close. On the normally day by day usage (ok I am lazy maybe) I only screw it up “hand tight”. Securely tighten it up, but not the whole way the thread can offer me. Maybe 80-90%. That’s makes me the usage easier and the reverse way to un screw is also easier. In this scenario I can still see a minimal gap in the backlit between Filter paper and the top of the tube ring. So because these cap is maybe 10-20 micron it is really really small, my “Bypass” theory maybe makes no more sense… But this observation was interesting enough to me for writing you to get a real definitive answer to it. Thanks to you! Many greetings from Weiterstadt in Germany. – From Dennis K.

The filter cap holds the filter tightly against the bottom circle edge of the chamber, insuring that all coffee is forced through the filter. The side holes of the filter cap are there because the pressing pressure will force a small amount of coffee to exit sideways above the paper filter. The side holes give this sideways forced coffee an escape where it will still drip down into your cup. Without the side holes a tiny amount of coffee might creep upward and along the bottom of the flange and drip outside of your mug.