Action, the height of the strings above the frets, measured at the 1st and 12th frets. Action at the 1st fret is primarily affected by the depth of the slots in the nut, while action at the 12th is affected equally by the nut and the saddle.

• Action and playability, Mya-Moe Ukuleles - link
• Lowering Action at the Bridge on Ukulele, by Frank Ford - link
• Checking Nut Action, by Frank Ford - link
• Lowering Action at the Saddle, by Frank Ford - link

• String Action Gauge, Stewart-MacDonald - link

  Measure string action (the height of the string above the fret) at any point by placing the gauge behind the string. The string height markings are at increments of .010" (ten thousandths of an inch). When the bottom of a mark aligns with the bottom of the string, that measurement is the string height at that point.

  The markings themselves are .005" thick, so referring to the top of a mark instead of the bottom adds .005" to the measurement. In this way the progressive string height scale measures in .005" increments.