From Latin 'instrumentum' (a tool), from 'instruere' (to build into) — something built into a process to make it work.
A tool or implement for delicate or precise work; a device for producing musical sound; a formal legal document; a means by which something is achieved.
From Latin 'instrūmentum' (a tool, an implement, equipment, furniture, a legal document, a means to an end), from 'instruere' (to build in, to equip, to furnish, to arrange), composed of 'in-' (in, into) + 'struere' (to pile up, to build), from PIE *strew- (to spread, to strew, to arrange). The suffix -mentum (Latin instrument-forming suffix) indicates a concrete result or means of an action: just as 'documentum' is what teaches (from 'docere'), an 'instrūmentum' is what equips or builds. The word's semantic range in Latin was broader than modern English
A 'financial instrument' (stocks, bonds, derivatives) and a violin are both 'instruments' for exactly the same etymological reason — both are tools 'built into' a process to make it function. The violin is equipment for making music; a bond is equipment for raising capital. Italian 'strumento' dropped the 'in-' prefix entirely, showing how far sound changes