Friction stir lap welding experiments using a heat-treatable aluminium alloy and detailed analysis of the welds have been conducted to understand the various hooking-related features and their effects on facture strength (sigma(Lap)). The effects of the 'penetrating depth' on the manner in which a sample fractured and on sigma(Lap) were first investigated. How rotation speed (omega) and welding speed (v) affect hook size (h) was then studied. Furthermore, lower stir zone size (A(B-SZ)) was measured and change of temperature in stir zone (T(SZ)) during FS was monitored for selected experiments. It was found that low penetration required low v for a sufficient bonding. For sufficient penetration, (omega, v)-A(B-SZ)-h relationships were presented. Measured data have suggested that the increase of AB-SZ as omega increases is the result of increase in T(SZ) and thus the stir zone plasticity. A rapid increase in h to a maximum value (h(Max)) when A(B-SZ) increased from a minimum value was identified. When h tended to zero, despite of the existence of an un-welded lap and thus a high stress concentration, sigma(Lap) was very close to sigma(Butt) (fracture strength of butt joint geometry). This is due to local bending and thus reorientating to reduce considerably the stress concentration. It was found that when h < similar to 30% of the plate thickness (t(Plate)), sigma(Lap) was not strongly affected by h. This surprising result has been explained by considering hook shape, hook discontinuity and FS softening that competes with hooking for the local deformation and fracture. When h > similar to 30% t(Plate), it started to invoke a significant effect on reducing sigma(Lap), due to the increasingly larger reduction in load bearing area.