In alkynes the hybridisation is sp i.e, each carbon atom undergoes sp hybridisation of one sand one porbital to form two sp-hybrid orbitals. Out of which one overlaps to form C – C ? bond and other overlaps with half filled orbital of H forming ?-bond. The other two 2p-orbitals remain unhybridised and form two ?-bonds via sidewise overlaping.Hence two ? bond and one sigma bond between C — C lead to cylindrical shape.