1, unit: kg, km and kW. ...

2. As unit vector: the unit vector of the third dimension in three-dimensional space.

3. when summing with ∑ k.

..............................

These usages are not absolute, but general habits.

K is different from m, the usage of m is strict, Mg is mg, mg is millions of grams,

Both Kg and Kg can mean kg, but kg looks uncomfortable.

In physics and chemistry, the absolute temperature scale must be K, not K;

In atomic physics and quantum chemistry, the energy layer of an atom must be K, not K.

In mathematics, I, J, K, L, M and N are usually used to represent counting.

Or as an upper subscript of a function or variable.

Supplement:

When foreigners learn English, when writing English letters, they are not as exaggerated as some teachers in China say, such as handwriting, style, inclination 15 degrees, hyphenation, abbreviation and so on. On the contrary, they saw several phenomena:

1, when children learn some letters, they learn to write Chinese characters like our children: horizontal and vertical.

2. The English written by foreign primary and secondary school students is very regular and neat. It emphasizes Chinese characters.

Square people, writing English, are rambling, scribbling, dancing, messy and incredible.

The more distinguished and prestigious people are, the more they like to write in all capitals in English when they speak.

Only when they are writing long paragraphs can they write artistic English.

So it's usually K, not handwritten K, not cursive K. ..

The use of flowers gives people the feeling of being flashy and hypocritical.