Modular assembly system, such as a system for door communication, comprising a support and at least one first and at least one second functional unit (3, 3a, 3b, 4), wherein the first functional unit (3, 3a, 3b) is an electrical installation apparatus (3, 3a, 3b), and wherein the second functional unit is a letterbox, wherein the functional units (3, 3a, 3b, 4) can each be fixed in a recess (AE, AB) of the support in a grid which consists of columns and rows, wherein a grid with square unit areas (QE1, QE2) is prespecified for the support, the respective base side of said square unit areas having a grid dimension (RM1, RM2) according to which the respective length (LE, LB) and the respective width (BE, BB) of the recess(es) (AE, AB) and the respective centre distance between the recesses (AE, AB) are dimensioned as an integer product of the grid dimension (RM1, RM2), wherein in each case main bodies of the functional units (3, 3a, 3b, 4) are mounted on the support by way of insertion into the recesses (AE, AB) of the support and are enclosed in the recesses (AE, AB) of the said support by the respective recess (AE, AB) in a manner matched in terms of shape, characterized in that the first functional unit (3, 3a, 3b) and the second functional unit have different standard dimensions, wherein the first functional unit (3, 3a, 3b) is an electrical installation apparatus (3, 3a, 3b) which is standardized in accordance with standard DIN 49075:1989-10, for which a standardized minimum permissible centre distance (MA min) is 71 mm, and the second functional unit is a letterbox which is standardized in accordance with standard DIN EN 13724:2003-5, for which a standardized minimum permissible dimension (EW min) is a minimum insertion slot height of 30 mm + 5 mm, wherein the grid dimension (RM1, RM2) is defined as a non-integer variable by which both a characteristic length or width dimension, which is not smaller than the standardized minimum permissible dimension (EW min) of the letterbox, and also the standardized minimum permissible centre distance (MA min) of the electrical functional unit can be divided without remainder, wherein the characteristic length or width dimension of the second functional unit is defined in that it gives the standardized minimum permissible centre distance (MA min) of the first functional unit when multiplied by an integer, and wherein the value of the grid dimension is defined as a rational, non-integer number which is a highest common denominator of the characteristic length or width dimension of the letterbox and of the standardized minimum permissible centre distance (MA min) of the electrical functional unit (3, 3a, 3b) since the grid dimension is 35.5 mm or is subdivided into an even finer grid dimension by means of division by 2, wherein the finer grid dimension is 17.75 mm, wherein the first functional unit (3, 3a, 3b) of the assembly system is of modular construction since its main body comprises a first part of which the basic shape is square and which can be fitted into recesses (AE, AB) of the support in a manner matched in terms of shape and which has a first rectangular side boundary contour, wherein the respective length and the respective width of the first contour are dimensioned as an integer product of the grid dimension (RM1, RM2) and wherein the main body comprises a second part of which the basic shape is square and which has a second, larger rectangular side boundary contour than the first part and, in the assembled state, completely or partially covers webs (6a, 6b) of the support between the recesses (AE, AB) and/or edges (7a, 7b) of the support at the recesses (AE, AB), wherein the respective length and the respective width of the second contour are dimensioned as an integer product of the grid dimension (RM1, RM2).