A drill string rod to form a part of a drill string, the rod comprising: an elongate hollow main length section extending axially between a first end and a second end; a male spigot portion provided at the second end having an externally threaded section and a non-threaded shank positioned axially intermediate the threaded section and a shoulder, the threaded section comprises at least one axially extending helical ridge and trough; the shank having a transition region positioned adjacent the shoulder at the second end; wherein a cross sectional profile of an outer surface of the rod at the transition region in a plane extending in a longitudinal axis of the rod is curved; characterised in that: the shoulder projecting radially from the main length section wherein an outside diameter of the shoulder is greater than an outside diameter of the main length section and the transition region of the shank; an outside diameter of the shank is equal to or more than an outside diameter of the main length section along a full axial length of the shank axially between the threaded section and the shoulder; and a curvature of the transition region axially closest to the shoulder comprises a first radius of curvature that is less than a second radius of curvature of the outer surface at the transition region axially closest to the threaded section, an outside diameter of the transition region over the first and second radius of curvature increasing in a direction from the threaded section to the shoulder. an elongate hollow main length section extending axially between a first end and a second end; a male spigot portion provided at the second end having an externally threaded section and a non-threaded shank positioned axially intermediate the threaded section and a shoulder, the threaded section comprises at least one axially extending helical ridge and trough; the shank having a transition region positioned adjacent the shoulder at the second end; wherein a cross sectional profile of an outer surface of the rod at the transition region in a plane extending in a longitudinal axis of the rod is curved; characterised in that: the shoulder projecting radially from the main length section wherein an outside diameter of the shoulder is greater than an outside diameter of the main length section and the transition region of the shank; an outside diameter of the shank is equal to or more than an outside diameter of the main length section along a full axial length of the shank axially between the threaded section and the shoulder; and a curvature of the transition region axially closest to the shoulder comprises a first radius of curvature that is less than a second radius of curvature of the outer surface at the transition region axially closest to the threaded section, an outside diameter of the transition region over the first and second radius of curvature increasing in a direction from the threaded section to the shoulder. the shoulder projecting radially from the main length section wherein an outside diameter of the shoulder is greater than an outside diameter of the main length section and the transition region of the shank; an outside diameter of the shank is equal to or more than an outside diameter of the main length section along a full axial length of the shank axially between the threaded section and the shoulder; and a curvature of the transition region axially closest to the shoulder comprises a first radius of curvature that is less than a second radius of curvature of the outer surface at the transition region axially closest to the threaded section, an outside diameter of the transition region over the first and second radius of curvature increasing in a direction from the threaded section to the shoulder.