A distributive lattice with zero is completely normal if its prime ideals form a root system under set inclusion. Every such lattice admits a binary operation $$(x,y)\mapsto x\mathbin {\smallsetminus }y$$
(
x
,
y
)
↦
x
\
y
satisfying the rules $$x\le y\vee (x\mathbin {\smallsetminus }y)$$
x
≤
y
∨
(
x
\
y
)
and $$(x\mathbin {\smallsetminus }y)\wedge (y\mathbin {\smallsetminus }x)=0$$
(
x
\
y
)
∧
(
y
\
x
)
=
0
— in short a deviation. In this paper we study the following additional properties of deviations: monotone (i.e., isotone in x and antitone in y) and Cevian (i.e., $$x\mathbin {\smallsetminus }z\le (x\mathbin {\smallsetminus }y)\vee (y\mathbin {\smallsetminus }z)$$
x
\
z
≤
(
x
\
y
)
∨
(
y
\
z
)
). We relate those matters to finite separability as defined by Freese and Nation. We prove that every finitely separable completely normal lattice has a monotone deviation. We pay special attention to lattices of principal $$\ell $$
ℓ
-ideals of Abelian $$\ell $$
ℓ
-groups (which are always completely normal). We prove that for free Abelian $$\ell $$
ℓ
-groups (and also free $$\Bbbk $$
k
-vector lattices) those lattices admit monotone Cevian deviations. On the other hand, we construct an Archimedean $$\ell $$
ℓ
-group with strong unit, of cardinality $$\aleph _1$$
ℵ
1
, whose principal $$\ell $$
ℓ
-ideal lattice does not have a monotone deviation.