Public educational buildings – such as schools, libraries, research centres and museum galleries – have complex and often conflicting requirements in terms of their programming and functioning. On the one hand, they need to provide open and equal access to knowledge to various categories of users. On the other, they have needs that might restrict or condition the arrangement of space, movement and various activities. At the same time, social and technological changes cause these typologies to change from within so as to include the idea of learning as a form of socialisation. These shifts imply complex or conflicting spatial, programmatic and organisational needs and point towards a hybridisation of strong and weak programme organisation (Hillier, Hanson, Peponis 1984; Hillier 1996). This paper looks at two public libraries in London: Kensington Central Library and Swiss Cottage Library. The questions studied through these libraries are: firstly, how these conflicting requirements of space, programme and use are manifested through their spatial structuring and social performance? Secondly, how do weak and strong programme aspects of these buildings influence their day-to-day functioning? Finally: what is the role of the space of these libraries in influencing the strengthening or weakening of the boundaries between these programmatic categories of activities? It is argued that although both libraries are similar in scale and programmatic description, they have a crucial difference: their spatial structure. This difference exposed the influence of the spatial manifestation of programme on the transpatial definition of programme. The combination of the position of activities in the spatial layout and the length of the description of such activities are pointed as fundamental aspects to be observed regarding the influence of programme in the actual use of space – especially the potential in generating unprogrammed social encounters. It is found that the KCL leans towards the strong and formal end of this programmatic typology, being a library of an academic character. The SCL on the other hand, intensifies the informal and weakly structured aspects of this typology, functioning as a library-community centre.