The shape of the symbols were chosen to be both familiar (0,1) and easily stacked vertically (see the next section).
Furthermore, the symbol Ti was chosen such that the right side of even numbers will have similar curves.
Symbol names were chosen to be easily remembered. Also consanant-vowel sequences were picked to allow easy (although sometimes awkward) stringing of syllables when pronouncing a number.
A vertical stacking of the base4 symbols is used to maintain a compact number representation. When a basebase42 number is decoded into base4, a top->bottom, left->right (eurocentric) scanning is used.
Obviously, one can compose a base42 symbol by recalling the base4 symbols.
The symbol composition is also meant to be hand-written quickly. Each symbol has either none or one discontinuities.
A disadvantage of both sets of symbols is the collision with existing symbols. Out of context the symbols may be misinterpreted. As this is already common with 0 and 1 in existing symbol sets, we treat this as acceptable.