217. A Riddle With Calculated Proportions🤔

RIDDLE 217:

It can be used to measure, and is part of a fish, when adjusting size up or down you’re actually doing this, It’s part of music,
It helps your voice; What five-letter word is your first choice?

Look for subtle clues in the background photo, title, and tags (tags are below)

ANSWER IS ON PAGE 2 HERE