The _listening post_ is not under the control of the commander of the support point but is in the service of the Intelligence Officer. It is furnished with a microphone for the purpose of picking up enemy’s telephone messages in the hostile line. The _observation post_ belongs to the sector and is used for purposes of observation and security. [Illustration: OBSERVATION POST] The _combat post_ is established to obtain flanking fire along the barbed wire entanglements or it is used as a bombing post when in close proximity to the enemy’s trenches. Sometimes a series of combat posts are connected and a new trench established in front of the firing line. This is called the observation line. It is best to avoid this practice as there is always the question whether to hold or abandon this line when the enemy’s attack develops. In either case there are grave disadvantages. If it is held you have a weak line. If you abandon it the enemy can use it for shelter. Advance posts give excellent service if not too numerous. If there are many of them they are a temptation for hostile raids. =Dimensions of Trenches.= These can be found in text-books on the subject. The general idea is that they are to be narrow and deep to prevent observation and to present a small target for high-angle fire. =Loopholes.= Loopholes are used in the ordinary life of the trenches for observation and sniping purposes; but against the real attack of the enemy the firing is done over the parapet. If it is necessary to meet the enemy in hand to hand combat the men climb up on the parapet. =Traverses.= Traverses are used to localize the effect of shell or grenade explosions, and to prevent enfilade fire. They are sometimes used in long stretches of boyaux or communicating ditches as emplacements for A. M. R. to enfilade the same if the enemy penetrates the position. =Transversal Lines.= Transversal lines are usually constructed between the first and the second lines and between the second and the third lines. They are short stretches of trenches parallel to the front for special purposes: such as disposing of machine guns and trench mortars in depth; bombing posts for rifle and hand grenadiers; depots and observation posts. Command posts are usually located in such small transversal trenches, a little in rear of the lines they are in command of. For instance, the captain in charge of a support point might be located in a transversal just in rear of the cover trench. =Support Trenches.= The second or support line is established at such a distance that the first line is covered by its fire if the latter is captured.