A cutting tool assembly comprising: a holder comprising: a holder front surface; opposing holder top and bottom surfaces (30, 32) and first and second holder side surfaces (34, 36) extending therebetween; and a recess, having a recess side surface, formed on the first holder side surface and extending rearwardly from the holder front surface, and a recess top surface adjacent the recess side surface and formed on the holder top surface; and an adapter removably secured to the holder, the adapter comprising: an insert clamping portion; opposing adapter top and bottom surfaces (58, 60), and adapter inner and outer side surfaces (62, 64) extending therebetween; and an adapter side plate protruding transversely from the adapter inner side surface, the adapter side plate having an adapter side plate rear surface adjacent the adapter inner side surface and an adapter side plate front surface; wherein: the adapter inner side surface abuts the recess side surface; the adapter side plate rear surface abuts the holder front surface at a holder front surface portion that is adjacent to the first holder side surface; characterised in that in a bottom view of the holder, the holder front surface portion forms an internal acute angle a with the first holder side surface; and in a bottom view of the adapter, the adapter side plate rear surface forms the acute angle a with the adapter inner side surface. a holder comprising: a holder front surface; opposing holder top and bottom surfaces (30, 32) and first and second holder side surfaces (34, 36) extending therebetween; and a recess, having a recess side surface, formed on the first holder side surface and extending rearwardly from the holder front surface, and a recess top surface adjacent the recess side surface and formed on the holder top surface; and a holder front surface; opposing holder top and bottom surfaces (30, 32) and first and second holder side surfaces (34, 36) extending therebetween; and a recess, having a recess side surface, formed on the first holder side surface and extending rearwardly from the holder front surface, and a recess top surface adjacent the recess side surface and formed on the holder top surface; and an adapter removably secured to the holder, the adapter comprising: an insert clamping portion; opposing adapter top and bottom surfaces (58, 60), and adapter inner and outer side surfaces (62, 64) extending therebetween; and an adapter side plate protruding transversely from the adapter inner side surface, the adapter side plate having an adapter side plate rear surface adjacent the adapter inner side surface and an adapter side plate front surface; wherein: an insert clamping portion; opposing adapter top and bottom surfaces (58, 60), and adapter inner and outer side surfaces (62, 64) extending therebetween; and an adapter side plate protruding transversely from the adapter inner side surface, the adapter side plate having an adapter side plate rear surface adjacent the adapter inner side surface and an adapter side plate front surface; wherein: the adapter inner side surface abuts the recess side surface; the adapter side plate rear surface abuts the holder front surface at a holder front surface portion that is adjacent to the first holder side surface; characterised in that in a bottom view of the holder, the holder front surface portion forms an internal acute angle a with the first holder side surface; and in a bottom view of the adapter, the adapter side plate rear surface forms the acute angle a with the adapter inner side surface.