A turbine disk and blade arrangement having a plurality of turbine blades mounted circumferentially around a disk. Each blade has a fir tree root which provides a series of fore-to-aft-extending projections and grooves. The disk has a plurality of radially extending posts which define fir tree recesses therebetween. Each fir tree recess also provides a series of fore-to-aft-extending projections and grooves allowing each fir tree root to engage with a respective fir tree recess. On each pair of facing sides of the fir tree root and the fir tree recess, one or more projections of the fir tree roots and/or the fir tree recesses are truncated to form fore-to-aft-extending cooling passages. The arrangement has lock plates which cover forward and/or rearward surfaces of the posts and of the fir tree roots and define feed channels therebetween which feed cooling air to the respective cooling passage.