Can I use both uppercase and lowercase letters to write C code?

The C language distinguishes between lower- and uppercase letters. Thus, printf is different from PRINTF, Printf, or PrIntF. It is said, therefore, that C is considered to be case sensitive. Both main and printf must be written in lowercase letters. For naming self-developed functions, you may use whatever case you consider appropriate. However, C traditionally is written primarily in lowercase letters. We describe situations when other than lowercase letters commonly are used, as the need arises.