Multi-line transmission (MLT) is a technique in which ultrasound pulses for several directions are transmitted simultaneously. The purpose is increased frame rate, which is especially important in 3-D echocardiography. Compared with techniques purely based on parallel beamformation, MLT avoids the need for reducing the transmit aperture and thus maintains a high harmonic signal level. The main disadvantage is that artifacts are caused by cross-talk between the simultaneous beams. In a conventional MLT implementation, simultaneous transmits would be spaced regularly in the azimuth and elevation planes. However, using rectangular geometry arrays, most of the acoustic side-lobe energy is concentrated along these planes. The results in this work show that the crosstalks can be pushed below the typical display range of 50 dB used in cardiac applications if the parallel transmit directions are aligned along the transverse diagonal of the array. Dispositions with 2 to 5 MLT for a typical cardiac 2-D phased-array were investigated using simulation software. Using the proposed alignment, the maximal crosstalk artifact amplitudes decreased 20 to 30 dB compared with conventional MLT dispositions. In water-tank measurements, side-lobe levels of a commercially available rectangular probe were 15 to 25 dB lower along the transverse diagonal, confirming that similar suppressions can be expected using actual transducers.