26. Clothes that, for the most part, contain shaatnez. Clothes that contain shaatnez most of the time may not be worn before they are checked (שו"ע יו"ד סי' ש"א ס"ב ). They are muktzeh on Shabbos and may not even be moved letzorech gufom umekomom (שו"ע או"ח סי' ש"ח סמ"ז ומ"ב סקק"ד ). If one wore such a garment without checking it and shaatnez deoraiso was ultimately found in it, he needs a kaporoh even if he sinned inadvertently, not knowing it needed to be checked (ט"ז יו"ד סי' צ"ט סק"ט ).
27. Significant minority. A garment of the type in which shaatnez is found in a significant minority must be checked lechatchiloh before use. If one wore it without checking it and shaatnez deoraiso was ultimately found, he needs a kaporoh (פמ"ג יו"ד סי' ל"ה בהקדמה להל' בדיקת הריאה ד"ה ועתה ). If one is in a pressing situation and it is difficult to check it, e.g., there is no time to check it before Yom Tov or a wedding, he may be meikel and wear it and check it afterward as soon as possible.
28. What is considered a significant minority? Some say in many areas of halochoh, 10% is a significant minority (משכנות יעקב יו"ד סי' י"ז הובא בדרכ"ת סי' ל"ט סק"ג ). Others say it does not depend on percentages; rather, a type of garment that shaatnez will inevitably be found after checking a significant number of garments is considered to have shaatnez in a significant minority, even if it is less than 10% (שו”ת שבט הלוי ח"ד סי' פ"א, ח"ה יו"ד סי' פ"ה סק"ה ).
29. Changing circumstances. Some say whenever circumstances constantly change, there is always an obligation to check and one may not rely on the accepted rate of 10% since at one point in time it may be less and at another, it may be more. In such a case, clothes need to be checked simply to determine the rate of occurrence of shaatnez (כך שמעתי ממו"ר הגר"מ בראנדסדארפער זצ"ל בעל קנה בשם לגבי מיעוט המצוי לענין טריפות העופות ). Thus, one should always check clothes of a type that the circumstances always change, even if at that specific time, the rate of occurrence is less than 10%.
30. Rare. Types of clothes that only rarely contain shaatnez are not technically obligated to be checked. Still, it is proper for a G-d-fearing person to check them; doing so is not a waste of time and money (הגריש”א ). If one wore this type of garment and shaatnez was only found later, he is an oneis and an oneis does not need a kaporoh (רמ"א יו"ד סי' קפ"ה ס"ד ).
31. One should not send a garment of the type in which shaatnez is almost never found to be checked. Doing so inundates shaatnez checkers with work at the expense of other people. This may prevent people from checking garments that need to be checked.
32. One-time check. An important point: although in other areas of halochoh, e.g., checking food for bugs, checking for treifos, etc., there are rules about rate of occurrence which impact the obligation to check, it could be that checking for shaatnez is different. When it comes to food, each time a person eats, he must determine if he must check the food, which could potentially be very burdensome. For clothes, on the other hand, a garment is checked once and determined to be kosher forever. This may be a reason to be machmir on shaatnez checking (השעטנז להלכה ולמעשה ).
