A two-way indexable cutting insert comprising: a regular tetrahedral basic shape defined by exactly four insert corner portions; and four insert face surfaces mutually intersecting each other, each insert face surface being connected to three insert corner portions and spaced apart from the remaining insert corner portion, the four insert face surfaces comprising: a first rake face surface and a first relief face surface that intersect to form a first major cutting edge; and a second rake face surface and a second relief face surface that intersect to form a second major cutting edge; characterised in that: each of the four insert surfaces has only one rake face surface (34a, 34b) or one relief face surface (36a, 36b), the four insert surfaces including exactly two rake face surfaces (34a, 34b) and exactly two relief face surfaces (36a, 36b); in a top view of the cutting insert, the cutting insert has a maximum cutting insert width, measured in a direction along the primary insert edge, and a maximum cutting insert length measured in a direction perpendicular to the cutting insert width; and the maximum cutting insert length is in the range W ≤ L ≤ W ∗ 1.5. a regular tetrahedral basic shape defined by exactly four insert corner portions; and four insert face surfaces mutually intersecting each other, each insert face surface being connected to three insert corner portions and spaced apart from the remaining insert corner portion, the four insert face surfaces comprising: a first rake face surface and a first relief face surface that intersect to form a first major cutting edge; and a second rake face surface and a second relief face surface that intersect to form a second major cutting edge; characterised in that: each of the four insert surfaces has only one rake face surface (34a, 34b) or one relief face surface (36a, 36b), the four insert surfaces including exactly two rake face surfaces (34a, 34b) and exactly two relief face surfaces (36a, 36b); in a top view of the cutting insert, the cutting insert has a maximum cutting insert width, measured in a direction along the primary insert edge, and a maximum cutting insert length measured in a direction perpendicular to the cutting insert width; and the maximum cutting insert length is in the range W ≤ L ≤ W ∗ 1.5. a first rake face surface and a first relief face surface that intersect to form a first major cutting edge; and a second rake face surface and a second relief face surface that intersect to form a second major cutting edge; characterised in that: each of the four insert surfaces has only one rake face surface (34a, 34b) or one relief face surface (36a, 36b), the four insert surfaces including exactly two rake face surfaces (34a, 34b) and exactly two relief face surfaces (36a, 36b); in a top view of the cutting insert, the cutting insert has a maximum cutting insert width, measured in a direction along the primary insert edge, and a maximum cutting insert length measured in a direction perpendicular to the cutting insert width; and the maximum cutting insert length is in the range W ≤ L ≤ W ∗ 1.5. each of the four insert surfaces has only one rake face surface (34a, 34b) or one relief face surface (36a, 36b), the four insert surfaces including exactly two rake face surfaces (34a, 34b) and exactly two relief face surfaces (36a, 36b); in a top view of the cutting insert, the cutting insert has a maximum cutting insert width, measured in a direction along the primary insert edge, and a maximum cutting insert length measured in a direction perpendicular to the cutting insert width; and the maximum cutting insert length is in the range W ≤ L ≤ W ∗ 1.5.