A **D-Brane** is a **strongly-coupled** sort of an ${n}$**-dimensional** generalisation of an **F1 String**. What are some of the simple properties of an **F1 String**?

**Weak coupling.**
**1-Dimensional.**

A **D-Brane** is **strongly - coupled, (i.e. with $g_s>1$)**; and is **$n$-dimensional**.

# The *need* for D-Branes

See, for example, **this article**. To summarise the main points, one needs D-Branes because:

**T-Duality**; **exchanges** Newmann (**Free**) boundary conditions with Dirchilet (**Bound to Branes**) boundary conditions, which means that Newmann Open Strings, in say, Type I String Theory, is by T-Duality, mapped to Dirchilet Open Strings, in it's T-Dual. **If** there are **open strings** (there are in the Type I), there **must** be **D-Branes.**
- If one sees the
**worldsheets** in a **Ramond - Ramond Potential**, it **doesn't couple**. To get around this, one must **accept D-Branes**.