A method of making one or more blade modules (12, 14, 116, 118) of a modular wind turbine blade, the method comprising: providing a mould assembly comprising a mould (62, 64, 70) having a mould surface (68, 72) shaped to define at least part of an outer shell of a first blade module (12, 116), and an elongate feature (66, 80) protruding from the mould surface, the elongate feature having a tapered end region (82a, 82b) defining an inclined upper surface (86a, 86b); arranging one or more first layers of fibrous material on the mould surface (68, 72) to form an outer skin of the first blade module (12, 116), the one or more first layers at least partially covering the elongate feature (66, 80); arranging an elongate spar structure (32, 92a, 92b) on top of the one or more first layers, the elongate spar structure having a tapered portion (46, 96a, 96b) that tapers in reverse to the tapered end region (82a, 82b) of the elongate feature (66, 80); arranging the tapered portion (46, 96a, 96b) of the elongate spar structure (32, 92a, 92b) on top of the inclined upper surface (86a, 86b) of the elongate feature (66, 80); arranging one or more second layers of fibrous material on top of the one or more first layers and on top of the elongate spar structure (32, 92a, 92b) to form an inner skin (28, 110) of the first blade module (12, 116); integrating the one or more first layers, the one or more second layers and the elongate spar structure (32, 92a, 92b) by means of a curable matrix material; curing the matrix material to form the outer shell of the first blade module (12, 116); wherein the elongate feature (66, 80) of the mould assembly forms a tapered recess (48, 120) in the outer shell of the first blade module (12, 116). providing a mould assembly comprising a mould (62, 64, 70) having a mould surface (68, 72) shaped to define at least part of an outer shell of a first blade module (12, 116), and an elongate feature (66, 80) protruding from the mould surface, the elongate feature having a tapered end region (82a, 82b) defining an inclined upper surface (86a, 86b); arranging one or more first layers of fibrous material on the mould surface (68, 72) to form an outer skin of the first blade module (12, 116), the one or more first layers at least partially covering the elongate feature (66, 80); arranging an elongate spar structure (32, 92a, 92b) on top of the one or more first layers, the elongate spar structure having a tapered portion (46, 96a, 96b) that tapers in reverse to the tapered end region (82a, 82b) of the elongate feature (66, 80); arranging the tapered portion (46, 96a, 96b) of the elongate spar structure (32, 92a, 92b) on top of the inclined upper surface (86a, 86b) of the elongate feature (66, 80); arranging one or more second layers of fibrous material on top of the one or more first layers and on top of the elongate spar structure (32, 92a, 92b) to form an inner skin (28, 110) of the first blade module (12, 116); integrating the one or more first layers, the one or more second layers and the elongate spar structure (32, 92a, 92b) by means of a curable matrix material; curing the matrix material to form the outer shell of the first blade module (12, 116); wherein the elongate feature (66, 80) of the mould assembly forms a tapered recess (48, 120) in the outer shell of the first blade module (12, 116).