Where is tail width measured and what aspects of a freeride board's performance does it influence?

Its measured at 1 foot off the tail, 305mm. On a freeride or slalom board a wider tail enables the back straps to be further outboard allowing you to use bigger or more powerful fins and larger sails. Can assist early planing and acceleration out of gybes and give better glide in the lulls.. A narrower tail can limit the upper sail size range of a board but can be faster and easier to control in chopper and windy conditions. It is a bit more complex than that as other factors come in to play especially if the board has cutouts but as a basic rule wider tail means better bottom end, earlier planing bigger fins and sails but can have control issues in the rough and get bouncy through gybes...narrower tail = smaller fins and sails, better control and easier to gybe in the rough stuff and can be faster.

traditionally nose and tail width are measured 300mm (1') from nose and tail. Don't know about freeride, but generally a wider board will be earlier planning. A board with more parallel rails will be stiffer, so for the same width in the middle and rocker line, a narrower tail will be looser

^^^ +1

and also how far forwards the cutouts extend = loss of lift when gybing

So Orange Whip, not just width....

Thanks gents. It's a pity that few board brands actually show tail width in their spec sheets.