  We show the analogue of Muhlherr's [Coxeter groups in Coxeter groups, Finite
Geom. and Combinatorics, Cambridge Univ. Press (1993), 277-287] for Artin-Tits
monoids, and for Artin-Tits groups of spherical type. That is, the submonoid
(resp. subgroup) of an Artin-Tits monoid (resp. group of spherical type)
induced by an admissible partition of the Coxeter graph is an Artin-Tits monoid
(resp. group). This generalizes and unifies the situation of the submonoid
(resp. subgroup) of fixed elements of an Artin-Tits monoid (resp. group of
spherical type) under the action of graph automorphisms, and the notion of
LCM-homomorphisms defined by Crisp in [Injective maps between Artin groups,
Geom. Group Theory Down Under, Canberra (1996) 119-137] and generalized by
Godelle in [Morphismes injectifs entre groupes d'Artin-Tits, Algebr. Geom.
Topol. 2 (2002), 519--536]. We then complete the classification of the
admissible partitions for which the Coxeter graphs involved have no infinite
label, started by Muhlherr in [Some contributions to the theory of buildings
based on the gate property, Dissertation, T\"ubingen (1994)]. This leads us to
the classification of Crisp's LCM-homomorphisms.
