Liquid mutation and Stop mutation are the rarest of the mutations in Sindarin. By the Third Age and beyond it is possible that these would not have been used at all and that soft mutation would be used in their place. If you wish to write in an older style, or more formally, it is still useful for you to know these.

Liquid mutation

Liquid mutation arose from the changes that occurred when Noldorin developed into Sindarin. This mutation was used on prepositions, prefixes and conjunctions that end in -L or -R such as Er- , Nel- and Or.
F, L, N, R, S and TH do not change.

Original letter	Changes to	Example
B	V	Or valrog = Over a demon
C	CH	Or char = Over a building
D	DH	Or dhaedelu = Over a canopy
G	'	Or 'ador = Over a prison
H	CH	Or chadhod = Over a dwarf
LH	'L	Or 'ling = Over a spider
M	V	Or vaeth = Over a battle
P	PH	Or pharf = Over a book
RH	'R	Or 'rovan = Over a wilderness
T	TH	Or thaes = Over a nail
(M)B	B	Or both = Over a puddle
(N)D	D	Or danwedh = Over a ransom
(N)G	G	Or gaur = Over a werewolf

Stop mutation

Stop mutation has a dubious existence due to a lack of consistent evidence, but is meant to be a variant of Soft Mutation and have occurred on words following the prepositions O(d), Ed and Ned. The letter changes below are those with the most evidence. Any other consonant can be assumed to change according to normal Soft Mutation rules.

Original letter	Changes to	Example
B	E/O B	O bastad = From a wedding
C	E/O CH	E chram = Out of a cake
D	E/O D	E dond = Out of a fist
F	EPH/OPH F	Eph fang = Out of a beard
G	E/O G	O gaear = From a sea
H	E/O CH	E chalf = Out of a seashell
LH	E/O THL	O thloer = From venom
P	E/O PH	E phesseg = Out of a pillow
RH	E/O THR	E thraw = Out of flesh
S	ES/OS S	Es sigil = Out of a necklace
T	ETH/OTH TH	Eth thaur = Out of a forest
TH	ETH/OTH TH	Eth tham = Out of a hall
(M)B	E/O MB	E mbahad = Out of a marketplace
(N)D	E/O ND	E ndôr = Out of a land
(N)G	E/O NG	E ngurth = Out of death