Side-sword

The spada da lato or side-sword is the Italian term for the type of sword popular during the late 16th century, corresponding to the Spanish espada ropera. It is a continuation of the medieval arming sword and in turn the predecessor of the rapier of the Early Modern period. Its use was taught in the Dardi school of Italian fencing, influential on 17th century rapier fencing.

They were ideal for handling the mix of armored and unarmored opponents of that time. A new technique of placing one's finger on the ricasso to improve the grip (a practice that would continue in the rapier) led to the production of hilts with a guard for the finger. This sword design eventually led to the development of the civilian rapier, but it was not replaced by it, and the side-sword continued to be used during the rapier's lifetime.

Although this particular type of sword is referred to in modern times as a side-sword, the name was not used contemporaneously to the sword in question (as far as current research of ancient fighting manuals goes). Although some early Italian sources use the term spada da lato, they are only talking about the sword that is at your side, and not just this particular type of sword.

Also of note is that as rapiers became more popular, attempts were made to hybridize the blade, sacrificing the effectiveness found in each unique weapon design. These are still considered side-swords and are sometimes labeled sword rapier or cutting rapier by modern collectors.

...
Wikipedia