A turbine rotor (34;3 5), comprising: a disk (37;200;300) rotationally disposed about a central axis (A;308), the disk (37;200;300) including a bore, a rim (72;203;302) and a web (70;202;306) disposed radially between the bore and the rim (72;203;302); a first rim portion and a second rim portion, the second rim portion disposed radially inward of the first rim portion; the first rim portion including a first blade post and a second blade post, the second blade post spaced circumferentially a distance from the first blade post, wherein each blade post includes one or more circumferentially extending branch elements, a tip, a base, a forward facing surface and an aft facing surface; the second rim portion including a first tab (92;210) positioned radially inward of the first blade post and a second tab (92;210) positioned radially inward of the second post; and a slot disposed circumferentially between the first tab (92;210) and the second tab (92;210), wherein the first tab (92;210) and the second tab (92;210) have an axial profile that includes a blade post transition portion, a slope portion radially inward of the blade post transition portion and a face portion radially inward of the slope portion, wherein the forward facing surface of each blade post has a first portion positioned in a region between the tip and the base and that extends radially inward from the tip to the blade post transition portion; characterised in that: the slot has a circumferential length equal to a circumferential length of the first tab (92; 210) and the second tab (92; 210); the first rim portion further includes a third blade post positioned circumferentially between the first blade post and the second blade post and radially outward of the slot; and the blade post transition portion is positioned proximate a radially most inward one of the branch elements. a disk (37;200;300) rotationally disposed about a central axis (A;308), the disk (37;200;300) including a bore, a rim (72;203;302) and a web (70;202;306) disposed radially between the bore and the rim (72;203;302); a bore, a rim (72;203;302) and a web (70;202;306) disposed radially between the bore and the rim (72;203;302); a first rim portion and a second rim portion, the second rim portion disposed radially inward of the first rim portion; the first rim portion including a first blade post and a second blade post, the second blade post spaced circumferentially a distance from the first blade post, wherein each blade post includes one or more circumferentially extending branch elements, a tip, a base, a forward facing surface and an aft facing surface; the second rim portion including a first tab (92;210) positioned radially inward of the first blade post and a second tab (92;210) positioned radially inward of the second post; and a slot disposed circumferentially between the first tab (92;210) and the second tab (92;210), wherein the first tab (92;210) and the second tab (92;210) have an axial profile that includes a blade post transition portion, a slope portion radially inward of the blade post transition portion and a face portion radially inward of the slope portion, wherein the forward facing surface of each blade post has a first portion positioned in a region between the tip and the base and that extends radially inward from the tip to the blade post transition portion; characterised in that: the slot has a circumferential length equal to a circumferential length of the first tab (92; 210) and the second tab (92; 210); the first rim portion further includes a third blade post positioned circumferentially between the first blade post and the second blade post and radially outward of the slot; and the blade post transition portion is positioned proximate a radially most inward one of the branch elements. the slot has a circumferential length equal to a circumferential length of the first tab (92; 210) and the second tab (92; 210); the first rim portion further includes a third blade post positioned circumferentially between the first blade post and the second blade post and radially outward of the slot; and the blade post transition portion is positioned proximate a radially most inward one of the branch elements.