The term "bread-and-butter letter" refers to a thank-you note or letter of appreciation written for routine acts of kindness or assistance. Some synonyms of this term include "appreciation note," "gratitude letter," "thank-you note," and "acknowledgment letter." These types of letters are often sent to express gratitude for small gestures such as holding the door, providing an introduction or referral, or offering kind words of support. Bread-and-butter letters are simple yet powerful, as they can strengthen relationships and build trust between individuals. Regardless of the specific term used, the sentiment behind a bread-and-butter letter is one of gratitude and appreciation for the small acts of kindness that make life a little brighter.