https://phys.org/news/2018-05-matter-antimatter-asymmetry-neutrinos.html

According to modern physics, quarks are the most important indivisible building blocks that make up matter. We know of six flavours of quarks: up (u), down (d), strange (s), charm (c), bottom (b) and top (t); each flavour also has its own antimatter counterpart (often marked with a dash above the letter, read as "bar"). Quarks are generally formed in quark-antiquark pairs. They are extremely sociable particles: almost immediately after coming into being, they bind into hadrons, or groups of two, three, and sometimes more quarks or antiquarks, bonded with gluons (i.e. particles transferring strong nuclear interactions). The process of combining quarks/antiquarks into complexes is called hadronization.

Unstable hadrons built from quark-antiquark pairs are called mesons. If one of the quarks in a meson is a charm quark, the particle is called a charm meson and is denoted by the letter D (or for the charm antiquark: D with a bar above it). A pair built of a charm quark and a down antiquark is a D+ meson, and one consisting of a charm antiquark and down quark is a D- meson.