Writing for Mallets
When writing parts for mallet instruments that need the player to hold two mallets per hand (like marimba/vibraphone chords) it is very tricky for the players to rapidly play passages that include constant switching between “black and white” and “white and black” note combinations in single hands.
This means if you for example want in one hand the notes of F and Bb together followed quickly by Ab and D in the same hand it will be very tricky for the player to achieve this: Both mallets are held in a V shape in each hand. The notes on mallet instruments are arranged like on the piano, with the front row including the “white keys” and a row behind that including the “black keys”. In order to reach for instance F and Bb together in the right hand, the player needs to bend his/her elbow outwards so the V shape of the mallets becomes more of a > shape in order to reach the notes. With Ab and D it is the opposite where the elbow needs to move inwards to create more of a < shape with the mallets.
Obviously, quick succession between these two configurations is practically impossible as the “way of movement” is very long. So be aware that these instruments can not be treated like “two fingers per hand” pianos when writing for them.