We describe a new, generally applicable strategy for the systematic construction of basis invariants (BIs). Our method allows one to count the number of mutually independent BIs and gives controlled access to the interrelations (syzygies) between mutually dependent BIs. Due to the novel use of orthogonal hermitian projection operators, we obtain the shortest possible invariants and their interrelations. The substructure of non-linear BIs is fully resolved in terms of linear, basis-covariant objects. The substructure distinguishes real (CP-even) and purely imaginary (CP-odd) BIs in a simple manner. As an illustrative example, we construct the full ring of BIs of the scalar potential of the general Two-Higgs-Doublet model.Everybody is used to the conventional way of setting up quantum field theory models: One picks fields in certain representations of symmetries and the Lagrangian is parametrized as linear combination of all symmetry invariant operators up to a certain dimension. However, if there are multiple fields with exclusively the same quantum numbers, these fields are physically indistinguishable, implying that they may be mixed at will, without observable consequences. On the Lagrangian level, such a mixing of fields does, in fact, correspond to a mixing of symmetry invariant operators, thereby parametrizing the Lagrangian in different ways. This arbitrariness in parametrization (or basis choice, in different words) must not affect physical statements derived from a model. Notwithstanding this, the presence of large basis change freedoms often obscures the physical properties of a model.In order to make the physical discussion as general and transparent as possible, it seems worthwhile to use basis invariant (BI) objects. An original arena for BI techniques was the detection of CP violation in the Standard Model (SM) [1] and extensions [2][3][4][5]. Here, a formulation in terms of basis invariants (BIs) immediately gets rid of spurious rephasings, thereby allowing direct access to physical properties of the model. Many more applications of BIs are conceivable and -ultimately -it should be possible to describe and relate all physical observables, say S-matrix elements, correlation functions etc., in terms of BI objects. Having such a formulation would be wonderful, but has to date not been put forward in general.Here we solve a major technical problem which arises as the first step along the way to any BI formulation: Given a theory formulated in an arbitrary basis, how does one obtain basis independent quantities in a controlled manner? Several different ways have been used to construct invariants in the literature (for a certainly incomplete list see e.g. [2][3][4][5][6][7][8][9][10][11][12][13]), none of which is entirely satisfactory for varying reasons. Several occurring shortcomings of previous approaches are at the same time advantages of our newly proposed method:• It is completely clear for us when we can stop looking for new invariants, i.e. when we have found a complete set of independent invaria...