Abstract. We study the relationship between min-plus, max-plus and Euclidean convexity for subsets of R n . We introduce a construction which associates to any max-plus convex set with compact projectivisation a canonical matrix called its dominator. The dominator is a Kleene star whose max-plus column space is the min-plus convex hull of the original set. We apply this to show that a set which is any two of (i) a max-plus polytope, (ii) a min-plus polytope and (iii) a Euclidean polytope must also be the third. In particular, these results answer a question of Sergeev, Schneider and ButkoviΔ [15] and show that row spaces of tropical Kleene star matrices are exactly the "polytropes" studied by Joswig and Kulas [12].The notion of tropical convexity (also known as max-plus or min-plus convexity) has long played an important role in max-plus algebra and its numerous application areas (see for example [3,4]). More recently, applications have emerged in areas of pure mathematics as diverse as algebraic geometry [7] and semigroup theory [9,10].Recall that a subset of R n is called max-plus convex if it is closed under the operations of componentwise maximum ("max-plus sum") and of adding a fixed value to each component ("tropical scaling"). A max-plus polytope is a non-empty max-plus convex set which is generated under these operations by finitely many of its elements; max-plus polytopes are exactly the row spaces (or column spaces) of matrices over the max-plus semiring. There are obvious dual notions of min-plus convexity and min-plus polytopes. Minplus and max-plus polytopes are sometimes called tropical polytopes 3 . The projectivisation of a max-plus polytope is the set of orbits of its points under the action of tropical scaling. Since any point can be scaled to put 0 in the first coordinate (say), restricting the polytope to points with first coordinate 0 gives a cross-section of the scaling orbits, and hence a natural identification of the projectivisation with a subset of R nβ1 . A subset which arises from a max-plus polytope in this way we term a projective max-plus polytope. In general, a projective max-plus polytope is a compact Euclidean polyhedral complex in R nβ1 ; it may or may not be a convex set in the ordinary Euclidean metric. Joswig and Kulas [12] studied the class 1 Email Marianne.Johnson@maths.manchester.ac.uk. 2 Email Mark.Kambites@manchester.ac.uk. 3 Typically one fixes upon either the "min convention" or the "max convention" and uses terms such as "tropically convex" and "tropical polytope" to refer to min-plus or max-plus as appropriate. A key feature of this paper is that we study the relationship between min-plus and max-plus convexity, so for the avoidance of ambiguity we will tend not to use the word "tropical".