The two letters that are both symbols for 1,000 are "k" and "K". In the International System of Units (SI), the lowercase "k" is used as a symbol for the prefix "kilo," which denotes a multiplication factor of 1,000. Similarly, the uppercase "K" is commonly used as an alternative symbol for kilo in many contexts. Both "k" and "K" represent the same value of 1,000. This convention is widely recognized and used in various fields such as science, engineering, technology, and finance.