Through the concept of ‘marginality’, ‘slit’ and ‘in-between’ as border, interstitial and imprecise spaces, and definitely dialogical, we reflect on the form, space and meaning of the ‘in-between place’ in Fontana’s Concetto Spaziale and in Siza’s Church, in a dynamic and reciprocal exploration between art and architecture, idea and its materialisation, tradition and innovation, order and transgression, harmony and conflict, as polarities made intentionally present in both works. Through the Concetto Spaziale we reflect on Fontana’s recognition of the ‘cut’, the slit or the perforation as a sign of liberated space, of the unlimited meaning of space, of the value of emptiness, of transgression and the known, and of the awareness of another dimension of an unexplored world, which emerges as another concept in art and as a new meaning in the cultural universe. Through the Santa Maria’s Church, a work with vernacular, modernist and expressionist roots, we reflect on Siza’s intrinsic reading of the architecture of a church as a religious meeting place, highlighting the importance of the marginality of the ‘slit’, the ‘forces of place’ and his ‘sliding theory’, which intentionally intersect and dialogue. Both are presented as dialogical fields that generate a subtly ‘restless’ or tense formal relationship, translated spatially and symbolically into a dynamic of transgressions, conflicts and ambiguities, yet order, balance and harmony are present in an integration of tradition, innovation and contemporaneity. In the marginality of its slits an inspiring ‘in-between place’ is built which assumes itself as the interstitial, dialogical and imprecise boundary in its multiple dimensions.