mano a mano; mano mano; man mano
As far as Italians are concerned these expressions (freely exchangeable) have only one use and meaning, the origin of which - most likely - has to do with the way in old times
people used to measure
short distances by using an open hand.
You put a hand widely open beside another end moving on as you take measures. From this habit in old times may have arisen the idea of following an event - shal we say - "step-by-step" (if you're measuring
long distances) ... or similarly "hand-by-hand" (if you're measuring, say, a cloth or a stick of wood, etc.).
By extension, every time an
Italian wants to mean following something very closely and at every new event uses this expression.
I mark on the
wall my child'
s height as (mano a mano) he grows.
Segno sul muro l'altezza di
mio figlio MANO A MANO che cresce.
I
mark on the
wall my child's height (continuosly) AS he grows.