And just a small amplification of HD's remarks. D stands for "Day" duty. At Heathrow Tower we have seven different day duties with staggered starting times from 0630 to 1530, numbered D1 to D7. The staff on day duties are from different Watches to the duty Watch, and provide relief breaks. The term "Spin" comes from the original title of the day shifts which was "Rotators", - i.e., you rotate onto another Watch. At Heathrow the term for day shifts is (confusingly), "Co-ordinators".