When we walk with a partner, we become a four-legged animal, with the accompanying gait possibilities.
Animal gaits are categorized according to how many beats the four footfalls make, four-beat (ambling), three-beat (canter and gallop), and two-beat (trot and pace).
Almost always in tango we want a clean two-beat walk, either pace-system (conventionally known as ‘parallel system’, deprecated) in which the mirrored feet move at the same time, or trot-system (conventionally known as ‘cross system’, deprecated) in which the feet move in diagonal pairs.
In tango, we don’t use the three beat gaits. Amble is used, rarely, as a dramatic and very special effect.