+## SUBVL - Sub Vector Length
+
+This is a "group by quantity" that effectively divides VL into groups of elements of length SUBVL. VL itself must therefore be set in advance to a multiple of SUBVL.
+
+Legal values are 1, 2, 3 and 4, and the STATE CSR must hold the 2 bit values 0b00 thru 0b11.
+
+Setting this CSR to 0 must raise an exception. Setting it to a value greater than 4 likewise.
+
+The main effect of SUBVL is that predication bits are applied per **group**,
+rather than by individual element.
+
+This saves a not insignificant number of instructions when handling 3D vectors, as otherwise a much longer predicate mask would have to be set up with regularly-repeated bit patterns.
+